Starting points:
1) Welcome back! 4e is very diff to previous eds, so you'll have to start from basics again.
2) You can download the starter rules and Keep on the Shadowfell for free from this site. do so. Read. Test play some of the encounters.
3) Buy the PHB (and if you're going to DM, get hte DMG and MM). You might also want to invest in the Dummies Guide to 4th edition, PHB2 and DMG2. Read. Lots.
4) Download the demo version of the character builder and mess around with it.
5) Read, contribute to the various forum threads here. Join some of the forum groups on the website.
6) Download and listen to the PvP podcasts.
7) Talk to the people you're going to game with; or, if you haven't found a group yet, find a group to play with and talk to them!
Good Gaming!