Given the following assumptions:
You have engaged in RPing before
You want to learn combat
Your friends have also RPed
You own a copy of the DMG
My recommendation is to not actually start out with an adventure. It sounds like your primary intent here is to learn the combat system. In the back of the DMG, there are descriptions of how to build a random dungeon and how to build random encounters. If you and four of your friends can create an adventuring party, you may learn the combat system (in quite an entertaining way, I must say, having done so myself) through a random dungeon and random encounters in said dungeon. Someone may choose to control the monsters every round, and it's worthwhile to have the duty of controlling them rotate around the people who are willing: that way everyone can start modelling in their head how monsters think and operate.
Once you've gotten a handle on the monsters and combat, then is the time to run adventures. My recommendation would be to use the chaos scar adventures and encounters from DDI, as they represent much later adventure design and this design will be similiar to that run in D&D Encounters at a game shop. Once you've run through those, test your skills with a fourthcore run of the "FourthCore Weekly Grind" which can teach you really important lessons about manipulating and using your environment that can be quite applicable in an Encounters game.