How to gain weight quickly?

would like to know has much has possible on how to gain weight and cheap, i have been told that i have a very fast metabolism,... i eat one meal a day and snack a few times anything else is a struggle , it is starting to get to me i thought about taking vitamins because i dont get everything from eating that i should so 4 days ago i started taking 1multivitamin 1 vitamin b and 1 omega fish oil capsule a day will this help , wud apppreciate any help thank you