I would start by rebuilding a simple engine like a old lawn mower, find one someone is throwing out
Then buy an engine stand, and a good tool kit, including torque wrenches and all the other stuff you'll need as you go
Then move on to a multi cylinder engine, again, find any old engine, maybe a cheap or free old V6 buick out of a commodore, pull it totally apart, then put it back together, you don't need to replace parts if it isn't going back into a car, think of it as a training tool, so the only real cost would be tools
Until then, get a floor jack, some engine stands, and do a full service on your car, buy some flush and do a engine oil and filter change, coolant, power steering, brake inspection and flush, transmission oil/fluid, and diff oil, inspect all belts and hoses, check all bushings and joints in the suspension and replace where required, if you don't thrash your car it should survive for another 100k km hopefully, not counting general maintenance and replacing old parts like sensors and other bits that typically go out on older higher milage cars
What the above does is introduce you to the "infernal combustion engine" and most of the other bits you'll need to address later
Be organised, lable EVERYTHING, you'll need space, a BIG white board to knock up your plans, goals, parts and most importantly the costs
Costs always blow out, work out how much you think you will spend, then double it, at least
Do all of this prior to pulling your engine
Your only 18, don't rush, think of it as your 2 year project of learning, only after all of the above, pull your engine
I cannot count how many people that have just "dug in' only to have a car that never runs again, or turns into never ending unfinished project
DON'T start buying parts for the LS build yet (apart from servicingit), wait until you pull it and get it cleaned and machined, that will let you know what stuff you actually need
You'll probably need a cheap reliable daily for a year once you pull the pin on the LS rebuild, there are always hurdles to jump, expect them and you will be fine, stuff NEVER goes to plan, think of them as learning experiences and stay positive
Always have plans, in 3 months have the car 100% services, in 1 year know more than you do now about engines than you do now, in 2 years know more, in 3 years........, in 5 years........
Always be realistic and add in "fudge " factors to time lines
Nothing good happens quick, you need deep time for the good to evolve
Buy once-cry once
Start a build thread with lots of pictures NOW and record everything you do, your servicing, your lawn mower build, your plans, goals and time lines
Remember this should be fun and enjoyable, if it isn't fun and enjoyable your going to hit a wall, hard, and lose motivation
Happy automotive journeys