What foods should I eat to gain weight?

I would really need more information before I could give proper advice for your question - however the way that people gain weight is to consume more calories than they burn. 

I wouldn't recommend eating an excess of nutrient-empty foods in order to do this (i.e. cake, cookies, ice cream).  You could try using nutrient supplements such as protein or soy protein drinks or products such as Ensure.

 Again, I do not have the appropriate amount of information to give you direct guidance on this.   I suggest you speak with a licensed nutritionist or a Registered Dietitian for proper advice.

Providing you've checked with your Doctor there is not an underlying disorder that would account for you being underweight such as a hormonal or digestive disorder then the healthiest way to gain weight is to build muscle.

Essential fats and Amino acids are the best way and hemp seeds and hemp oil are a good source of both of these. Hemp oil has a good balance of omega 3 and 6 essential fatsand the protein has a good balance of amino acids.

These are the raw ingredients your body needs as building blocks for cell membrane production and building muscles.

If your metabolism burns calories unusually quickly then try adding something like organic coconut fat to your food - although this is a saturated fat it is more easily converted to energy and obviously quite calorific without the detrimental effects of sugar.

More info from London Nutritionist  at www.optimumnutritionists.com