I find 3.X DnD easily lends itself to rule-layering and can easily decend into petty fights about "Well, is that
really what the rules say? I think they mean this..."
Still, I think a real foundational agreement that the group needs to make is Rule 0 "The Dm makes the final call."... that, and that FUN is more important than getting every rule right. If you're a fair DM, and the group REALLY does care about FUN more than rules, then you're sitting pretty good as a group.
Maybe having a sit down chat with the group, reaffirming the group's commitment to fun, fair play and the giving of "authority" to the GM might be a place to start.
1) Reaffirm your relationship with your friends -- tell them that you've loved playing with them so far and want to continue with them.
2) Thank them for the trust that they've given you to moderate your current campaign. Tell them how excited you are to be preparing these adventures for them
3) Admit to them that you feel somewhat under-qualified to DM a 3.x game. Admit that they seem to know a lot (more) about the rules and that you sometimes feel intimidated by this.
4) Share with them honestly your frustrations about how your game sessions seem to be getting derailed by arguments/ discussions about the rules.
5) Share with them your vision of how you would like to see the gaming sessions run. Share with them your primary goal is to ensure that they have a great time roleplaying, interacting with the world you are creating for them.
6) Share with them that you understand that during game play there will be questions about how certain rules come into play. Tell them that if it won't take too long and you can find the specific rule easily in the books that you have on hand, you'd be happy to look it up and make a ruling. However, if the rule is not readily accessable during gameplay, tell them that you will make a ruling that you think will be fair
for this gaming session, and will further look up the rule before the next gaming session. This, you hope, will help game play continue smoothly.
7) Ask them if they are willing to trust you to make judgements regarding the rules, with the understanding that if they were running the game, they might rule differently. If they agree, then ask them to affirm Rule 0 - "If I make a judgement call on a rule, I need to know that, even if you disagree with my ruling, you will accept it and together we can move forward."
Invite them to give you suggestions about GMing and to offer to help you to find specific rules
outside of the gaming session... preferably, after the gaming session.
9) Assure them, that when you make judgment calls regarding the rules, you're not looking to the best interests of your NPCs, your pet villians, or your ego. Affirm that when you make judgement calls, you are looking to the best interests of your gaming group as a whole and to what will make the gaming experience fun for all.
10) Tell them that you are excited to see how their characters will develop and grow and you help them explore this word you have created.
11) Affirm the relationship once more, sharing with them that you hope this group will be gaming together for many more sessions.
Whew... wow... that became a lot longer then I had planned! Anyway, hope this helps!
BTW... welcome to the Inn!
Ayrn