I know I'm not playing really tense, but it feels like im not relaxed enough. I started noticing my arm would get tired and sore from playing for like, 10-15 minutes. My strumming arm's shoulder started to get sore and so did my forearm. My picking hand's wrist is hurting too.

I know I'm supposed to relax my right arm (strumming) and bring it up slowly to reach my guitar. But the problem is I guess my guitar is too high when I'm sitting down and I think I'm using too much effort to raise my right arm to reach the string. My fore arm is resting on the body of the guitar. Is this normal? I'm playing electric by the way.

I'm still a beginner (8 months or so), so is this just my muscles still developing? I've never had a teacher and I'm trying to correct my technique through GuitarPrinciples by Jamie Andreas .

tl;dr arms hurt from playing, can't relax
Watch the first part of John Petrucci's "Rock Discipline" video. He demonstrates a lot of non-guitar warm-ups that should help you.
Sometimes concentrating on minimising tension can actually produce tension. I'd say completely forget about it and just approach the guitar with your muscles completely relaxed. If you start to tense, back it up and think about playing tension free.
The thing is that it feels like im playing tension free already. I know I'm playing with tension because I can't go too fast and my muscles start to get sore real fast.
I've had a recurring case of RSI for years that has only just been identified as such. I've been given strengthening exercises for the wrist by a physio that basically involves gently lifting a can of beans. I've also bought a powerball which the jury's still out on but the blurb on their website describes it as little short of a miracle worker.
Got it:
Pacifica 604w, Hohner G3T, PRS SE Soapbar I, Schecter Ultra I
Fender 'evil' Twin, Marshall 2554 combo
you may also be putting too much strain on your entire arm when youre playing. any sort of fast picking techniques should be executed using the movement of just your fingers. also try slowing the riffs down to build good foundation muscles. and when you do feel tension, make a break and drink some water, hydrating your muscles is the key to avoid tension and cramps.

good luck