It's fairly simple you might want to read up on this a little more, mainly the stickies. You can either keep the stock amp connected and use the speaker level inputs on the ms8, or you can remove the stock amp, recode to hifi, and use the RCA inputs on the ms8. If you decide to recode to hifi I have a harness that will work for you.
http://www.e90post.com/forums/showthread.php?t=903918
To power the amp you should get a 4 or 8 gauge wiring kit, youll have alot of extra power wire but thats never a bad thing. You'll also need the connectors for the underseats subs and the power connectors for the battery. I think you can get both of these from technic, or if you search around you can find the part numbers and get them from the dealer.