Having just done this myself for the first time (and feeling dumb about wondering whether I was doing things right, and still not sure so don't hold me to this!), I at least know that it might be a little challenging to find the right battery. Don't know about 318's, but my 325iC takes a super-weird sized battery, one that's not carried by AutoZone or PepBoys or just about anywhere else. I was able to find it at BatteriesPlus, saved at least $70 from what the stealer would have wanted. They also installed it for free, not that installation is difficult, but it was very nice in that they disposed of the old battery for me.
You might want to have someone check whether you have a particular problem causing the battery to have failed, like a bad alternator. Supposedly that's what AutoZone did for me (my battery was able to charge at least briefly, enough that after a jumpstart and running the car for 30 minutes I could get one or two starts out of it, so I drove the whole rig to AutoZone) so you might want to take it there and try to buy a battery from them after the diagnostic, and plead ignorance when they find out they don't carry the right size.