Hey there, bought my first electric back in Jan, acutally these past 2-3 months i've only started playing more, bought an acoustic as well.

Basically im doing it all by myself, bought some books, atm im going page by page through the book learning the first 3-4 frets of each string, their notes + music theory associated, some simple songs, having a blast so far. I know i aint near rocking but I mean everyone starts somewhere, so I'm hoping just to keep up the little small basics till i get better.

Anyway so far I've been just working things out, catching onto some small hints wherever i can eg: fretting hand, the thumbs best on the back of the neck between the first(index) and second(middle) finger for mobility/ease of moving from frets. I was wondering if anyone had some more little nifty tips for fretting, picking, position, posture etc that could probably correct little bad habits I have(since im not using a teacher and I basically have no friends i can jam with and gain experience of).

Anyway ive been playing my new acoustic recently and I've found my back gets pretty tired/sore after awhile. Am I sitting in bad posture? I basically sit with both legs over the side of the bed, none raised up, acoustic on the right leg, inclining slightly forwards in front of me to make picking easier. MY right arm/picking arm is pretty high up, its a pretty big bodied guitar, is this what's making my back ache? Or is it my posture? I do sit with my legs forward, back twisted slightly to the right in order for picking/general playing, reading the instruction book im learning, is this is?

Should i be sitting up dead straight? Or leaning forwards abit(which i find makes my back feel less tired/sore). I havent been playing for hours so yeah, why is my back getting sore so fast(10-25 mins)?

Also does anyone have tips on up-picking? I find down picking quite easy, but when i up pick it has a habit of getting caught on the strings/not as fluid. I find alternating the angle abit when up-picking easier, but somehow i cant get myself to naturally do it versus downpicking where my hand automatically just finds the best way to get that pick over the string with as minimal contact. Do i just need more practice/accuracy up-picking?

And most importantly before i develop bad habits: should I be relying more on my actual thumb/index holding the pick to be moving and picking? Or should i be more of holding my fingers still and using my wrist motion to pick down/up/ etc?

SOrry for the long post, any helps appreciated!
P.s My back is more sore in the top middle part than the sides, it definitely reminds me of when you sit upright for too long with no back support or whatsoever, is this something like painful fingers, that i will just have to harden my resistance to? Or bad posture on my part?

Any pictures/sites/videos that show the best way to sit when you have no back support when sitting eg: how i sit on the edge of my bed, or whatsoever would be great guys