As you can see from my member title, I am also a newbie. I have an unamped, unsubbed, setup LOL.
I'm pretty sure your 325i is a single din. So get a new single din stereo. I recommend an Alpine, Kenwood, or Pioneer.
After you do that, get a wiring harness for your car. (If you buy it from
Crutchfield, it should come with them)
And I think BMW's need a special antenna harness also.
Basically, you plug the harness into your factory slot, and then crimp or solder connections to the harness form the new HU, then you install it.
Pretty easy, just search on google for your specific car, there should be at least one!