“How Can I Gain Weight and Keep it? Where do I start?”

Hello I am 25yo female who is 5’5 and always at or right under 100lbs. I want to gain some weight and be able to keep it. I want to do this not only for my own health but for my self image concerns. I eat protein (majority chicken) , some form of potatoes, and pastas for the majority of my meals. I don’t eat veggies unless they come on my plate at dinner. I love protein shakes and fresh smoothies. I’m not that active and rarely work out, but my most recent job kept me on my feet and casually exercising (standing and walking for 8 hours). 2-3 years ago I fell into a depressed stage and lost the body figure I thought was healthy and I haven’t been able to put that weight on since and I really don’t know where to start. I would love to see 15-20 pounds added to myself. Appreciate anyone who consults!