For the first 3-9 months or so you'll be using an infant carrier, get one with a removable base and use the center belt to secure the base only. You can use them in the outer positions too, but if you use a rear-facing carrier with a base, you have to move the seat up a bit because they take a lot of room. Otherwise you can belt just the carrier in, but angles are important with rear-facing and it's a PITA to buckle and unbuckle just the carrier. If you put the carrier in the center I found you didn't have to move the seat. We had a Graco carrier with an adjustable base, I found putting it in the middle with it adjusted all the way up would have the seat at the right level.
Our daughter outgrew her carrier around six months, I replaced it with a different graco seat, also in the middle rear-facing, didn't have any problems.
This winter I used a Britax Marathon 85 forward-facing on the passenger side, it was a pain to get it in with the seat belt because then you use the little clip that comes with the seat it's difficult to adjust the slack in the belt... you'll see. Once it's in it's not so much of an issue, the Marathon 85 has an odd belt path that doesn't really work too well with any car (and I've used it in three).
Congrats!