Here are a few guidelines that will help you get your diet in order.
1) Get at least 1g of protein per pound of bodyweight, and I would recommend getting them from lean sources such as poultry, eggs, lean beef, dairy products, fish, protein powder.
2) Make sure to get an adequate amount of essential fatty acids, and I would recommend getting ~25% of your daily cals from fat. the majority of your fat intake should come from healthy sources such as fish oil, olive oil, flax oil, nuts, natural peanut butter, etc.
3) You can fill in the rest of your daily cals with low glycemic index carbs such as oats, veggies, sweet potatoes, etc.
4) As far as the amount of calories you will need to consume daily, I would recommend slowly increasing cals in small increments each week until you are gaining a small amount of weight each week (~.5-1lb. per week). You can use a site like www.fitday.com
to help you track cals.
For example, if you are currently eating 2500 cals and not gaining weight, then you would increase cals to maybe 2800 for a week. Weigh yourself at the end of the week and see if you gained any weight, and if not then repeat this process until you are gaining. I would recommend that you weigh yourself at the same time each week (first thing in the morning is probably the best time).
5) Drink about a gallon of water a day.
6) If you want you can purchase some whey protein to use in your post workout shake along with some high GI carbs such as dextrose/maltodextrin.
Hope that helps somewhat, and feel free to post your diet if you like