Dude!
First of all, entire careers have been devoted to master this technique. Business have been created to do nothing but this service. With the utmost respect of all the posters here, a few advice posts are not enough to make you a pro. Sorry. But maybe your asperations aren’t so lofty.
The first thing to remember is that not all two (or more) images can be merged together. If your images have compatible perspectives (the angle that you’re looking at them with), focus (how sharp or fuzzy are the “outlines”), lighting (how strong are the shadows? what direction are they going?) and image fidelity (pixels per inch and color depth). If all these things aren’t there to begin with, even the experts will struggle to get them to look believeable.
Option 1:
Check the lighting cast. For instance, some outdoor scenes have a cooler (or bluer) light than indoor scenes, which tend to skew warmer. You can often get pretty darn close by a combination of “Curves”, “Color Balance” and “Hue/Saturation.”
Now the part that you’re really asking about: isolating a portion of an image and transposing it onto another image.
Start by making a rough selection of your subject that needs to be merged onto the other. Make it rough, with plenty of room around the edges.
Copy.
Go to the image with the background your subject needs to be merged onto.
Paste.
Use the Transform command (Command-T for Mac, Control-T for Windows (I think)). Grab a corner, hold down the Shift key (to constrain) to scale and rotate it to it’s proper size. Get it right the first time: every time you resize or rotate, you lose image fidelity. Oh, and obviously, don’t scale things larger, or it will start looking semi-pixelated. (If this is the situation, you may want to consider scaling the background down to match the size of your subject.)
Save now (as a .psd format with layers).
Now you do the detail work and mask away the areas around your subject you don’t want to see. If the subject has fairly hard edges, that’s pretty easy. The tools the other posters have mentioned (lasso, majic wand, etc.) are decent beginner tools but depending on your image, may leave you wanting. You get better control with a masking techniques. Extensis sells a product called Mask Pro that is supposed to be excellent for this process. Photoshop also has an “Extract” feature under “Filter” that is designed to do this. Personally I haven’t used either because I created my own techniques that have always worked for me, so if you want more info on how to use those, you’ll have to read the manual.
I isolate an area by using a layer mask. (at the bottom of the Layers palette, second button from the left: it’s icon looks like a dotted circle inside a rectangle.) The way it works is like this: if an area of the mask is white you can see that part of an image. If it is black, that portion of the area is hidden. If it is some level of gray, it will look somewhat transparent, depending on how dark the gray is. (It can be difficult to explain in a monologue fashion on a board, you have to experiment with these tools to understand how they work.) So I will want to paint over the area I want to isolate with the brush tools. Using white for the area I want to keep and black for the areas I want to remove. These tools work best because (a) you can isolate areas a little bit at a time (I’ll bet more than once with the lasso tool, you got part or most of the way done and then made a mistake and it ruined your entire selection, right?) (b) You can used hard and soft-edged brushes to closely follow the in- and out-of-focus areas of your subject. © you can save your work in process, close the file, quit the program, come back to it later and you can pick up where you left off. As you are isolating the area with the layer mask, you will be able to see your background layer behind it.
Option 2
Pay me to do it for you. (Prices quoted depending on intricacy of image)
