These PianoTV videos/posts will help give you an understanding of what sonata form is, and what sonatinas are. To view the text-based explanation and screenshots, check out the post here. You can go as far as the explanation of the exposition/first movement (you do not need to read/view the entire thing yet.)

Once you've finished with that explanation, we're going to dig deeper into the exposition itself. Check out this post (explaining sonata form in the context of a Mozart composition) and/or the video below. You only need to watch through the explanation of the exposition.