installing a whammy bar?

It depends on what kind of guitar you have and how it was designed. If, for example, you have a stratocaster with a fixed bridge (no whammy bar) it would be quite a lot of work to install a tremolo bridge (whammy). You would have to make a cavity in the back of the guitar to accommodate tension springs and so forth. It CAN be done, but my advice would be to expand your horizons, save up and buy another guitar that does have a whammy bar so you don't run the risk of losing the tonality of your current guitar by heavily modding it.

Also, floyd rose bridges are good for whammy, but would require even MORE modifications and are an absolute pain to tune correctly if you play in different tunings.

Well, it depends if your guitar came with a whammy bar thing built in it (like a place to screw in your whammy bar), because some guitars, such as Gibson, don't install things for whammy bars. And I've never heard of anyone installing a whammy bar into their guitar, and I don't know if it would work, because whammy bars have to do a certain thing to your strings in order to make the cool sounds. But you can ask like, a guitar specialist, or someone at a guitar place, like Sam Ash or Guitar Center, and see what they say.

Yes you can have one installed. Doing so requires you buy a new bridge (tremelo) and either have it installed or you can mess it up yourself if you have the proper tools and skill. The body of the guitar needs to be routed for the new bridge to fit and a new nut installed if you want locking. Floyd Rose has instructions and routing templates here.. http://www.floydrose.com/instructions.html

The bridge and professional installation will cost hundreds of dollars (depending on bridge model). So find a bridge, and get some prices on installation, then after that look to see if you're better off getting a used guitar with one already installed as a cheaper alternative.

the guitar i have came with a whammy bar

and right by the bridge (the place where strings go in)

theres a little hole and u just twist it in to there