I think it's really important to start simple and slowly get a hang out of things.

Especially when players are new, everyone getting familiar with the rules of the game is the first priority at the beginning. It helps a lot to play a game where you are already very familiar with the rules. I'm not a huge 3.5e fan, but I know the rules so that's usually what we use when I start new campaigns.

Keep your goals simple. Don't try to start with a level 1 to 20 world traveling campaign, but start with simple one shot dungeon crawl adventures, one at a time. You can make NPCs from earlier adventures reappear later or have a villain for an idea you have for later make a short appearance in the adventure you are currently preparing. Or have the PCs go to locations where they have been before.
The adventures are still self contained and it's no problem when new players join or others leave, but it will be a nice touch when the player remember "hey, we know that guy!" or they see what has happened to a goblin they let run away two adventures ago. But as a beginning GM, first stay away from long plots and continous campaigns. Since the players are new, one shot adventures that take place in the same general area and have them keep using the same characters will be more than enough exitement for them.

And don't be afraid to do things that have been done a million times before. The first one or two times you do them as a player are still entertaining.