Gaining weight~

Hi! I'm new to the trying to watch my weight/calories thing. I've always been really, really small. I was about 60 lbs until I hit middle school, and even then I didn't really put on much more until I got into high school. I went for my freshman year, and I was a good 115 pounds average. When I started to do homeschooling I slowly started to lose my weight. Then my muscle mass. I was going to the gym pretty regularly for a while, and then I lost motivation and stopped. I read ALOT, and I don't have a job right now. So I usually sit unless my younger sibling is home and even then I sit a lot. When I started dropping weight I hardly noticed, until I stepped on the scales. I have a really, really tiny frame, small shoulders, ankles, wrists. So I don't look sick or anything, I just don't want to drop to underweight and then get sick. I'm going to start working out again with my mom, who had more trouble losing weight. I've read all kinds of things about heavy lifting/rep failure, protein shakes and muscle mass building shakes, etc... But I wanted to talk to people who might be going through or have gone through something similar! So, my question is, how do I go about start gaining back all of this weight I've lost? Any suggestions or advice is really appreciated!
Oh, nice to meet everyone! c: