You could really go either way here. If you join a gym, they will definitely get you into shape. They know how to push people and they will do what they need to do with everyone. Even if you needed to stop, the great thing about training is that the people that go there won't make fun of you or anything. Very helpful people.
If you want to make sure that you will be in shape beforehand, then just get on a structured eating plan and exercise regularly. It is all up to you. Either will work so whichever sounds better to you is what you should do. Just make sure you are having fun, and everything else will fall into place.