dont listen to any of the advice given here (because it's quite ****) just eat six small meals a day, go with moderately high carbs, protein and fats behind that, so your ratios of cals would look like this 50/30/20 (carb/protein/fat) you should find out what your maintinance cals are then up that by 500, if you see no weight gain in seven days up it by 250 and keep doing so until you start gaining. heres a sample meal plan:
meal 1 - bowl of oats with cinnamon and splenda, glass of semi skim milk with a scoop of whey
meal 2 - canned tuna with low fat omega 3 rich mayo on two slices of wholegrain bread
meal 3 - grilled chicken breast with mixed vegetable cous cous
meal 4 - protein shake made with semi skim milk
meal 5 - protein source such as, lean beef, salmon, chicken etc, loads of veggies such as, spinach, brocoli, peppers etc and a baked sweet potatoe
meal 6 - pot of 0% fat greek yogurt with two table spoons of natural peanut butter
you should also start lifting some weights 3 times a week, you could follow this programme
http://forum.bodybuilding.com/showthread.php?t=998224edit - with your meals you should be eating every 3 hours, so if you start with breakfast at 7:30 am then the next meal would be at 10:30am and so on...