Several people have asked, "Could you talk about how you begin a drawing? How do we know where to start?" It's a very good question. So often sketchers gets frustrated because suddenly here they are on the left side of the page and there's no more room, or the drawing looks misshapen, and they can't figure out why. We all have those problems from time to time. But what I want to say up front is that "Most errors that we think are in drawing actually begin with errors in SEEING."
So, before we ever set our pencil to paper, we need to stop and look at our subject with an analytical mind. Can we imagine some simple geometrical shapes into which it fits? And then can we subdivide those shapes to get a better idea of the placement of different elements that make up the subject?
Here's a pot with two different kind of plants in it that I decided to draw from life last night:

and here's the drawing I made from it, from a slightly lower angle than the photograph:

It seems like a fairly complicated subject. I needed to figure out how to fit it onto my rectangular sketchbook page. So the first thing is to look a little more closely. What do we see in the subject? Well, we've got two fairly equal areas of foliage, and a large pot below them. Here's how that translated to the page:

We're not all that interested in the pot, so we make a decision to crop out most of it - and I've also made the neck of it smaller and the pot a bit narrower. But we can see that these three elements together: two rectangle of foliage, and part of a pot beneath -- will make our drawing taller than it is wide, so we decide to orient our sketchbook page as a "portrait" view rather than "landscape" (tall rather than wide). This way most of the subject ought to fit -- if we plan it correctly.
So now we want to make a few marks on our page that give us some guidelines for placement. I'm telling you exactly what I did when I began this drawing, because I had to go through these steps of thinking and marking my page. Otherwise I'm quite sure I would have had trouble.

I always look for shapes that are going to help me create a structure upon which to "build" the drawing. So here, I decided to draw a few curves that indicate the open oval of the rim of the pot. Before I did that, though, I needed to figure out approximately what was the center of the composition, and place my oval accordingly on the page. The blue rectangles that I visualized when studying the real-life subject are the key. Basically, I saw that I had two roughly equal lengthwise rectangular sections of foliage, and would need to leave an area about half as high at the bottom, for the pot. The oval of the pot would fit within the edges, because it's narrower than the spread of the foliage.
It didn't bother me that I wouldn't be able to get all the begonia foliage and blossoms into the drawing, because I like my drawings to be cropped by the edge of the page. Then I drew the two curves for the sides of the pot.
With this, or any other subject, I might also make a few little marks on my page that indicate key points in the subject -- for instance: the topmost point, the top and bottom of a container, the sides of the composition. I find these by studying the relationship of the objects I'm looking at to one another. Where is the centerpoint of the composition? It's often NOT the most interesting object, but you may tend to start there and then find you can't fit everything in. Where's the top and the bottom? If Objects B and C form the sides of your composition, is Object A located halfway between them? If not, how far is it? How much of Object D can actually be seen behind Object A? Does the top of Object A line up with the top of anything else? A great deal of drawing has to do with learning to see these relationships accurately - and over time, it will become almost automatic.
Obviously, the more practice you have, the easier it becomes to judge relative shapes and sizes of things. Is the plant taller than it is wide? Or does it take up a shape that's a narrower, shorter rectangle? You can use the trick of holding your pencil at arm's length to make relative measurements of your subject. Squint at it with one eye, and line up the top with the top of the begonia foliage. Mark the bottom of the dark sweet potato foliage on the pencil with your thumb. Now, without moving your thumb, rotate the pencil so that it's horizontal. Is the width of the pot greater or less than the height of the foliage? If they're equal, that would indicate a square format for those elements. And so forth. Learning to judge the relative shapes and sizes of what you see is one of the most important skills you need in drawing, so it's worth practicing it, and checking yourself.
Plants are great to practice on, because they're a lot more interesting than cups and saucers and bowls and bottles, and a lot more forgiving than a face or a human or animal figure. Combining them with the "harder" elements of still lives - various ceramic or glass objects, for instance - gives you practice in seeing and drawing ellipses in perspective and other forms too. And you know what? Those objects don't have to be drawn perfectly. What you're aiming for is a lively drawing, not academic perfection, and plants help with that sense of life. If you look at my drawing and the photograph you'll see it's not 100% accurate, and that doesn't matter.
Similarly, sketching a landscape can give you practice seeing and drawing shapes and their relative sizes. But try not to make yourself crazy by drawing complicated city streets, or making portraits of your friends right off the bat. Pets are notoriously hard to draw -- start when they're sleeping! So, begin somewhere manageable. There's a reason for all those still lives of fruits and bowls, or pictures of simple objects like chairs and shoes and books. I like this artist's Instagram feed because he does precisely that: he draws simple scenes from his daily life every single day. But he's a very fine printmaker and painter as well. Drawing is his foundation.

Once I had the pot indicated on the page, I began with the sweet potato vine foliage, from left to right -- but it doesn't really matter. I knew it was going to essentially "fill" the oval of the pot. What you need to do is take a minute to study the size of a representative leaf, and how the leaves are formed. Once you've placed that leaf in your drawing, all the other bits of foliage will be sized and placed relative to it.
Every plant -- every tree, vine, bush or flower -- has a typical growth habit, and you need to know what it is before you start putting lines down. I didn't know how these grew. I saw that the leaves have five parts: the large central leaf which is an oval, wider in the center, tapering to a point (most of the time), with deeply rounded divisions between it and the two horizontal side leaves. Then there are two little leaves at the back. Look first, study, get the shape in your head intellectually -- you might describe it in words like that to yourself -- then draw, and you'll suffer less! The tricky thing is that all the leaves which lie in this fairly flat plane are at different angles. But you will be able to discern quite a bit of repetition and see that most of them are simply rotated from your "model" original.
The begonia presented a different problem - it's not radiating out on a flat plane, but growing upright. Anytime we've got plants with stems -- that includes trees! -- we need to indicate the structure first. So what I do is to mark those stems before we draw a single leaf:

And then, as we draw, we "hang" all the leaves and flowers off that structure, just as nature does.
I like the analogy of a string of pearls lying on a table. If we try to draw each pearl individually, we'll probably lose the shape of the whole strand. But if we indicate the string first, even though we can't see it, and THEN draw the pearls, we'll have a much better result.
The begonia leaves, too, have a characteristic shape, and once I had figured that out, I could start adding them to the drawing.

The blossoms came last.
I hope this was helpful. It's just an example, but this basic method of looking, analyzing, and reducing to basic shapes is what I always use. Happy drawing!