It's awfuly hard to help you with so little information about what your trying to do. Some more specifics woudl realy help, and letting us see your origionals and where you are in the process would certinly be beneficial to your getting a real response.
As general rules of thumb when I'm working with compiling a couple images into one, I start by making sure that my selection is realtivly smooth. For instance if you're taking one person out of a photo and putting them next to another group of people and you use the magic wand you'll normally get a really clunky choppy selection that will be hard to blend in. When working with organic things in your pictures, make organic selection areas - by hand. grab a free hand selection that follows around the forum you want, giving a bit of space.
Now, getting it into your new image. It all depends on what you're trying to do. Sometimes I like to put the selection on a new layer and use opacity. Or blur the edges of your selection with a blue or smuge tool.
There's all sorts of tips and tricks to do various things. If you have no clue how to do what I've sigguested, or your still not quite sure, I reccomend a book. Working visually generally requires some visual aids. This book starts off assuming you know praticly nothing about photo editing working it's way up to some advnaced techinques. It also includes photoshop elements, which is where the screen shots come from. Photoshop is a really intensive program to get into for the beginner, and working with it's sister app - elements can be a much nicer way to get started.
Anyways, the book! It's called 40 Digital Photo Retouching Techniques by Jack Lee. The isbn is 89-314-3502-9. It should be fairly inexpensive to pick up from amazon, it's listed as $17 on the back or the book. Definatly reccomended while you're learning stuff.
art.googlies.net - personal website