Does it shift? Can you go from park to N, R, or D? Or is it stuck in P? It sounds like you can put it in D and it just doesn't do anything. Make sure it is actually shifting..
From what it sounds like, you need to check your trans fluid. If it is low it will not work right.
If your trans fluid is good, and it is actually shifting out of park and into other gears, then you need to take to a trans shop. This isn't something I'd recommend the average person try to repair. Even i don't work on my transmission. But check the fluid first, and make sure your shift cable and linkages are good, allowing it to shift to other gears