Gil has already been explained above.
You've got Tifa now, where are you at right now? What I'd do is, as soon as Tifa joins the party, fight some enemies at the Train Graveyard east of the train. Not so much for experience YET, but for gil. What I would do is buy enough Restore Materia for all three of them. (They already said how to equip Materia so if you have a Restore Materia already - I think you should - equip it on someone and then battle for enough gil to get two more, or at least one so there's multiple healers.) And maybe another Lightning. That might be all I'd go for for now coz they don't give that much gil yet.
Restore Materia is the better way to heal, and if it is "linked" in a slot with All, you can heal everyone with one spell. In battle, it will only work once per battle. (Until it levels to 2-star, then it'll work twice per battle, and so forth. This is the All I am talking about, Cure has unlimited uses. That is, until you run out of MP of course.)
Also, most towns have an inn you can sleep at for a pretty low amount of gil in relevance to where you are (and in a few cases, for free) so if you get low on MP, rest there. While you are in Midgar I'd grind more for gil than experience. After leaving Midgar, then enemies will start to give decent experience for leveling at "freedom" moments. And do NOT fight the Midgar Zolom until you have over a thousand HP.
If you are on Sector 5 Reactor, you won't have any opportunities to restore MP (except by Ethers if you have any) but you should be fine if you don't screw around.
Also, use multiple save slots for this. Keep a save file back at the last town that has inns and shops, then save under a different save slot when you are in a dungeon and somewhere you can't backtrack, or as easily.
I'm guessing you know how the random encounter system works. I would recommend putting Barret in the back row (under Order or Formation, select him and hit A twice), since he has ranged attacks meaning his attacks will do the same amount of damage even if he is in the back row. (Back row takes half damage from physical attacks)
Also, you will get a party member named Aeris later. At one point (not super far away but there will be quite a bit before that) she will be party leader, and when that happens, equip her and the other character that isn't Barret with Lightning Materia. (And if you don't have two Lightnings, then make the second one Fire or Ice, because there will be a boss that physical attacks will not reach.)
When you get the Enemy Skill Materia, always have it equipped it on someone. It will make you learn many useful skills.
When you first start to encounter forests on the world map (which is further away than that boss I mentioned), there is a particular enemy you will encounter in the forest sooner or later. Defeat her and then answer the following responses:
Not interested
Petrified
Wait a second
That's right
Let's hurry
And do NOT use the save point, it's a fake. I think if you screw this up you don't get another chance.
In the first area with forests, the town might not have an inn but enter the tower southeast of the town and there will be a free inn. (You are not obligated to do anything there until way later in the game, you'll know what I mean when the time comes)
At some point you will have to do a dolphin jump. Don't move around for this; just do two jumps and you'll get where you want to.
This is way late in the game but do not forget this: In Temple of the Ancients and Gaea's Cliff, you will find Ribbons. These are very important accessories as they block all debuffs. Temple of the Ancients Ribbon is through the 5 spot in the clock room, and Gaea's Cliff is, in one of the earlier rooms, through a wall on the far right side (check a youtube video if you're not sure). Both areas are unrevisitable so do NOT miss these Ribbons or the final boss will be difficult. To get any more you either have to do something very difficult at Gold Saucer or morph a Master Tonberry in the final dungeon (Yuffie's Ultimate Weapon makes this a lot easier; once you get the submarine, look up how to get the Ultimate Weapon).
As soon as you get the Barrier Materia (it is in a shop at one of the towns about halfway through the game), equip it to level it up so you can eventually use Wall, reducing all physical and magic damage you take by half. This will, again, be useful for the final boss. Have Destruct Materia equipped for that as well. Again, much later than where you probably are.
When you go to Wutai at the far left continent, have lots of Potions and Hi-Potions on hand.
Near the end of the game, you will enter a subway tunnel. Do not run forward yet, run backwards until you reach the end and get W-Item Materia. There is an exploitable glitch that can multiply any item you can use in battle, provided you have at least two in your inventory already.