Gaining weight as vegetarian

I want to gain weight so badly but i dont know how. i'm vegetarian. i dont know what to do anymore because nothing is working for me. im 17 now and i had the same weight since middle school which is 35 kg/77 lbs. and yes, i know im really really underweight, but i dont know how to change it. can anyone help me with that? ... and i dont want to go to gym if it's possible to gain weight without going there.