Guys, How To Do Proper Pushups? and other few queriess

i srsly dont know how.......... like i tried and the posture was kinda off.... pls help me. also im alone so no mate to help me.

i used to go to gym back then, life became stressful hence resorted to kinda few exercises. heard pushups and sqauts r best for weightloss but how tf will i do when idk. also, gym trainer of mine made me do standing pushups while leaning on to wall....dunno if thats fine. this fat n man boobs omg...'

also pls suggest me a proper diet, plsss

1. Basic details – age 18, sex M, height 5 9, weight70-75, and fitness level bad.