Im 15 and really skinny :(?

Basically I am really skinny, you can see my ribs etc and my arms + legs look like twigs. Summer has arrived and I'm afraid to wear T-shirts because my arms are so thin and I'm embarrassed. I want to put on weight and muscle but I never seem to be able to, I think I have a fast metabolism. After browsing through other questions, all people say is "go to the gym" or something. I would look stupid working out as I look like a skinny twig. Are there any basic foods that I should eat and are there any exercises that I can do in the home without big weights,etc? I also can't change my diet "To much" e.g going on some programme that changes my entire diet. I just need to know are there any basic foods or exercises that will start to help me and make me "less skinny"? Please don't say "go to the gym" as it really doesn't help. Also can you suggest any clothes to wear that perhaps cover my arms as I don't want to wear jumpers in the summer. I've also barely hit puberty. Thanks