Two popular resolutions without a doubt have to be: Learn something new and get into shape. There is no better way to accomplish both of those by starting to train in a Jiu Jitsu program.
A lot of people who don't practice have the idea that a BJJ gym is comprised by a bunch of "meatheads" that just want o beat people up but that couldn't be further from the truth. After you start you will be surprised and shocked to realize the average practitioner is simply a normal unimposing person just like yourself. The second thing you will be surprised about is how much of a sense of community and family there is amongst the members. The nice thing about that is that all these people become your informal support group to keep you focused on your new years resolutions!