So I'm playing this while its being repaired. What are some must knows?

There aren't many must-knows, just pay attention to the explenation you will get on how materia works, so you understand how to combine materia, understanding how this works will allow you to create very useful materia combos. The main sidequests are the "Golden Chocobo Sidequest" and the "Emerald & Ruby Weapon Sidequest", both of which you can leave to the end of the game, so don't worry too much about them now. Most of the Summons, Lv4 Limit Breaks, and ultimate weapons aren't missable and can be obtained towards end of the game, so enjoy the game for now and decide when you get more than half way through disk 2 if you want to google up the locations of these things.

You can totally miss Barrets, Vincents, Cait Sith, and Tifas ultimate weapon if u dont know what your doing

My memory might be rusty, but as far as I know you can go to Lucrecia's cave (for Vincent's) and Bone Village (for the key to Tifa's) at any point in Disc 3. Barrets is missable, but anyone who looks for chests would always find it. Cait Sith's is probably the easiest to miss, but if you do a thorough search you find it.

You have to go to Lucrecia's cave twice. First time in the second and second time in the third.
I'm pretty sure you get Tifa's by playing the piano.

Where are these?

Barret: At the end of Disc 2, in Midgar, climbing up the tower where Hojo is.
Vincent: Lucrecia's cave, visit her with a green, black or gold chocobo. She will give you his weapon, and after you've killed a certain amount of monsters, she will give you his Lv4 Limit Break.
Cait Sith: At the end of Disc 2, search the lockers in one of the top floors of the Shinra building. (If you notice when you go there in Disc 1, Cloud will see it, but doesn't pick it up)
Tifa: In Disc 3, you have to escavate in bone village the Sector 5 key in Bone village, with that you can walk into Midgar and head to the Wall Market (were you buy get the cross dressing stuff) and enter the tent with a sign that says "ITEM". Before there was only a computer and there was nothing to do here, but in Disc 3 you get her weapon if you examine the inside of the tent

To find Barret's, you gotta have him in the group the moment you pass by the chest's location else it's not going to be there.

oh, fdidn't know that, I always had him in my party, so I never had to face that problem

Yeah I run with cloud barret and tifa. What materia should I be looking for? Who should I give what? I understand the system but Idk what works best with who.