Good points from Caslon, especially about not making it too arduous at first. Can't tell you how many people I know who start getting in shape, and for a month or two really go at it, don't change other habits, and end up getting discouraged at the huge amount of effort needed to make little or no result - and they give up. Balance your goals between realistic and ambitious.
I also think it's a bad sign that you have a specific time frame. Getting healthy and in-shape should be a permanent change. First, you set yourself up for failure by saying "I want to look good by swimsuit weather," and second, even if you reach that goal, you may pat yourself on the back in the summer and then slowly quit your efforts, regaining all your losses.
I'm not a fitness expert, many others on this site have tons of experience, some of it professional. But as someone who made a life change a couple of years ago to get in shape, I was successful at sticking with it by being careful about my goals and ambitions. I doubt I'll ever reach an ultimate goal, it's a constant work in progress.
Oh... and find great music, as Caslon said.