5 kilograms within 3 months sounds reasonable. I'm assuming you want to build muscle so you're gonna have to start eating more. For a good baseline of how many calories you should be eating multiply your weight (in lbs) by 20. Try to get in at least 1 gram of protein per lb of bodyweight. You're also gonna have to lift weights regularly. It's all about consistency. Check out bodybuilding.com for some good beginner weightlifting routines and more information on nutrition.