If you already only eat one big meal, you could try eating little and often. Small pieces of fruit, a few nuts maybe, the odd bowl of cereal/pasta, canned fish, chicken. Etc. Just make sure your're getting your hitting your daily allowance (2500 to remain as you are, less to shrink, extra to grow) and getting all your essential vitamins etc and you should be fine. If you're serious about extra muscle, you'll be needing about 3000 calories and lots of protein. You might want to consider a powder if you can't stomach the thought of loads of tuna/egg whites.
As for exercises, the classics 'power lifts' will probably do you fine. You probably already know how to bench press, but you probably should go along to a gym so an instructor can show you the best form. You should also learn how to squat and how to dead lift - those three exercises alone should be enough to help you start developing muscle, but if you don't learn them properly they can cause all sorts of problems.