Finding PHP code for plugins

I’ve tried the search but not even sure how to search for what I’m wondering.

I’m completely new to WordPress (about a weeks experience now). I’ve been playing with a lot of things so there are a few issues here and maybe I could be helped with them all.

I’m adding some plugins that I’ve found on www.remarpro.com but need help on adding them to a page using code instead of the admin panel. For instance:

I have a static home page, newly created called page-index.php. I’ve added the recent posts widget <?php the_widget( 'WP_Widget_Recent_Posts', $instance, $args ); ?> and it works. Now I need to limit the number of posts it shows and can’t find documentation on doing that.

I also need some education in making pages. I’ve created a bunch of pages specific to what I want – because I didn’t want the default page with the content.php part added. I could only edit content.php once and need more pages then that. Hope that makes sense on what I’m trying to say there. Anyhow, I’ve downloaded plugins and wish to add them in these pages as I had with the recent posts above – but haven’t a clue what exactly that piece of code would be called or where to find it.

Lastly, and this kind of goes in with the last thing I asked – is there a good site or doc that would show how WordPress will put together a full site using the different php files that it comes with? I get the header/footer part but can’t see how the rest works and more importantly how to go about changing how I do that. If I click on a category for posts, it shows content.php for each and every post – I’m not sure why it does that.

I realize I’m asking a lot here but i’ve honestly tried searching and reading for this stuff. I’ve learned a lot from it, but sadly nothing specific to what I’m actually doing the research for. Any help would be appreciated, even if it’s just a link that will show me what I need to learn so I can answer myself.