Let's start with the B's - Boots (well Broken in), Backpack, Bag (sleeping) and Buddy - a knowledgeable companion is an immense resource.  Of course, there is your Body to consider as well - what kind of shape are you in?

Where are you going to be backpacking?  Environments vary considerably  between, say southern Arizona and the Olympic Peninsula in Washington and require significant differences in equipment and strategy. 

Have you any experience with hiking in general?  If not, begin with easy day hikes, gradually extending them to overnights, and proceeding from there.

Learn about the Ten Essentials.  Learn navigation skills (especially how to use a topo map) and first aid.  Learn to build a fire and when a fire is feasible and when a fire is dangerous.  Have the means to shelter yourself if necessary.  This can be no more than a simple lightweight tarp

Purchase gear gradually and thoughtfully.  Borrow or rent first, if at all possible.  As Rob6 states, this is a lifelong quest.

Welcome to the club!  Many wonderful times await!

Superusers do not speak on behalf of REI and may have received
one or more gifts or other benefits from the co-op.