This probably belongs in the MMA Newbie section.
Anyway like Intermission said, keep your hands up and keep your eyes open. One huge mistake is when someone is about to get hit, they close their eyes. This usually leads to them getting caught somewhere along the way in a flurry. Just train to always keep both hands up and eyes open. If your right hand is throwing, your left hand is up guarding. If your hands aren't punching, they are protecting your face.
For losing weight, I would do a lot of cardio exercises. You can lift low weight if you want but I would stay away from then. Make sure you are burning more calories than you are putting in. Eating protein will help your workouts.
There is a bunch of workout systems for losing weight. A good one is called cross fit training. Here is a link about it.
Remember to have fun but train hard! Good luck!