So I'm 6ft 3
Weight is around 75 - 80kgs
I really want to get bigger. I have a high metabolism and it's real hard to gain weight. I just want to do some easy things this year while i'm finishing school.
I've pretty much had the same build my whole life, tall, skinny. pretty fit.
No money for the gym yet though.
I've been doing push ups / sit ups etc every day.
I heard eating alot of protein helps, but it's hard for me as i dont buy / make my meals yet.
Any advice will help, thanks!

A tub of whey protein is $15-20 at your local grocery store, save your lunch money kiddy. Lift other household objects that weigh a bit...what i use to do back in the day.

I'm honestly the same height and weight as you. I can gain muscle and fat but not much at a time. Try eating a lot of meat and beans before or after working out. I recently invested in a curly bar and some weights and I use it three days a week. I also do pushups, crunches, and chinups three days a week (all in one routine). Try not to do too much cardio since you're metabolism is really good. I've heard somewhere the more body fat you have the easier it is to gain muscle.

Start eating alot of high protein foods, Eggs, Milk, Cheese, Meat.
Lift weights, high reps, low weight.
As said above take supplements.
Whey protein after you work out, and before you go to sleep at night.
Also take creatine 4 times a day, it will help you work out harder and longer. It also helps muscles retain water which will make you bigger.

High weight heigh reps
If you wanna put muscle on fast, search german power training

basically, you workout till you go catatonic (about 45 minutes of workout)
You then stop and eat alot ofhigh protein food with some carb

within a week youll have significantly increased muscle size
alot of this will be swelling, but after two weeks, what was originall swelling, will be pure muscle

You aren't that smart are you, high reps high weight? Tell me when your able to do that.

Even though you have an awesome user name(Silmarillion used to be my favorite book). What you just said is pretty much impossible. You can't do high reps of high weight as you just run out of steam. If you could do that then it would be ridiculously easy for everyone to gain muscle. Maybe if you are on roids you can try that out though...