How to gain weight??!

Lauren • Got my rainbow on September 26th. 🌈👶🏼 👼👼👼
Im 24, probably about 5'6 and weigh 100 lbs. and I keep losing. Ive always had a weight problem and I can't seem to keep the weight on when I do gain. I've been thinking about eating protein bars daily. Anyone else know a good suggestion?