Nutrition-wise, us a food diary to figure out how much you're eating and drinking each day. Increase by ~300kcal/day. Weigh yourself once a week, aiming to gain about 0.5lb/wk; 1lb/fortnight; 2-3lb/month (take into consideration that your weight can easily fluctuate by 5lb/day, so weigh yourself at the same time of day on the same day of the week in the same conditions -- first thing in the morning when you've just done your business in the toilet, haven't had anything to eat or drink, and are naked is most reliable -- and look for a general trend rather than exact numbers each week). If after a month you haven't gained 2lb, increase calorie intake by 200kcal/day. If after a month you've gained more than 5lb, decrease calorie intake by 200kcal/day.
Use your brain: eat meat, fruit, vegetables, whole grains, nuts, eggs, dairy and water; while nature isn't always better than human intervention, the less processing food has undergone, usually the better. If you really want to use your brain, assume that I'm not a reliable resource and do some further reading.
