This is the best way I’ve found to start a drawing, and it’s pretty easy too. All you need to do is cut a rectangle out of a piece of cardboard (like a shoebox lid) and use it to visually crop your subject.
Store-bought viewfinders work too. I’m just cheap.
Move the viewfinder around until you’ve found the best composition possible and tape it in place. Then draw the same size of rectangle on your paper.
When you look through the viewfinder, everything within the frame is what you’ll draw, and you can ignore the rest. Not only will it improve your compositions, but it will also help with the next few steps in the drawing process.