two things. to remember to breathe when you're playing guitar, it seems easy enough. but if you kinda work yourself out of breath just playing you'll have less room to work with singing. so work on that by just remembering to always be breathing when you're playing. but also you want to work on putting air into your vocals, stand up to sing don't sit down and compress your diaphragm.. ideally anyway. but singing and playing and remembering the lyrics and remembering to breathe is hard enough. but hitting and maintaining pitch is infinitely harder. so work on your do re mi's and really drawing out each note/pitch it's hard to train your ear being self taught and only playing guitar, so just make sure to always tune it up and play as much as possible.