In photoshop, go to file - open and select your illustrator file. In the following popup window, adjust your resolution to 300dpi and hit open. It'll take a moment as photoshop rasterizes your vector file, and then it's open. If your file opens and it doesn't include your entire composition, that's because it opens only what fits on the artboard of your illustrator file - just go back to illustrator and resize your composition so it fits on the artboard, save it and then open again in photoshop.
At this point, you can add other files and play with blending modes & opacity. Use the magic wand to select areas on your illustrator drawing - it's really easy for the wand to select areas since the vector drawing is so clean and has distinct edges.