one thing that wasn't immediately obvious to me about DPs, but makes sense if you think about it, is that even if you* have the right technology, you can't sign one if either party is in a war at the time. makes sense, you both need to be at peace to make that "i'll defend you if in the future somebody comes to try to beat you up" promise.
the biggest help to me when learning civ4 (and even now, after playing for ages), is the F12 key. that opens up the Civilopedia. once you get used to it you can click on a unit from the technology listing to check what it does and what it upgrades to, etc.
it's a really nifty game but that's a very steep learning curve, that's for sure! especially for diplomacy. it drove me bonkers at first and i thought i would never understand that part! sometimes i still don't, but it's one of my favorite parts of the game now, it adds flavor and makes things challenging, trying to balance out making as many people happy as possible ... well, as many people "not out to get me" as possible *giggle*.
good luck, have fun, and welcome to CFC
*that's actually "even if you or the other guy has the right tech". for tech trading (alphabet), money trades (currency), DPs, etc, either party knowing it is enough, you don't both need to know it. in case you wondered. most days are "free potentially useless information from KMad day" around here.