Body is slim. barley any muscle. i get tired easy & feel weak. what can i do to beef myself up? gain muscle

Eat Protein: It would help if you returned with a more specific question that included your height, weight and the food diary showing what type and how many calories you consume. If you are not eating 65% of your calories as protein you may not build much extra muscle.

Weight training: Builds strength/muscle mass combined with a well balanced diet.Hire an experienced trainer at first,so you learn proper technique.