wow this is my fav game so I can give you some good tips
I think ppl explained how to get yuffie and vincent so I'll go to general tips

FF7 is based on the materia system so, you must level your materia to go through the game smoothly. what you must be careful is that many materia have a negative and a positive side effect. For example, you can equip lightning, and it gives you +5 magic, for example (I don't remember exacly the numbers) but your strength will decrease in 4. So build your materia having that in mind (don't put a lot of summon/green materia in the same character) Other materia, like purple, blue and yellow don't have these drawbacks, so equip freely.

About aeris, she has the best limit breaks in the game, and, since she dies, you better get her to the last limit to take advantage while she's still alive (sounds mean right? haha). Easy trick to gain limit breaks: They are 7: 1-1, 1-2, 2-1, 2-2, 3-1,3-3 and 4. For example, to gain limit 1-2, you must use limit 1 10-15 times. To gain limit 2-1 you must kill 40-60 enemies, and this repeats until limit 4. to get those you need especific items for each character.

my final tip is to choose your favorite group while you play the game because in a certain part of the game (CD 2 and so forth) items get expensive, and you won't have money to buy weapons, armor and accessories for all characters.

That's it, sorry for the long reply, got a little excited hope it helped