There's a large amount of tools for modding FF7 available on this forum, and thankfully most of them are created in a very userfriendly way that even non-programmers can easily make use of them. For what you want to do, you should definitely get Hojo, ProudClod, WallMarket and Makou Reactor, all of which can be found in the forum's "Tools" section.
Wallmarket is for editing all sorts of stuff related to your own party, like properties of magic spells, equipment, materia, and items, and initial equipment and stat growth curves of your party members.
Hojo is for editing enemy stats like HP, MP, strength, magic, elemental and status resistances & weaknesses, items that can be stolen/dropped/morphed etc.
ProudClod is for editing enemy attacks and AI scripts, and opens up a sheer unlimited potential for designing your favorite battles, although editing AI definitely requires some learning until you'll get the hang of it (took me a couple years to understand most of it, lol). You can give enemies new attacks, have them attack multiple times in a row, have them perform counter-attacks etc.
Makou Reactor is the tool for editing the field files, and with it you can do A LOT. This is where you can decide the contents of treasure chests and Materia, or place entirely new ones in places you like, but it goes much farther than that. You can also edit NPC dialogue, create new NPCs, change the rewards of minigames (Fort Condor, Battle Arena etc.), which monster formations appear as random encounters in a field and at what probability, change the music that plays in a given field or in certain battles, or even do some event editing like creating entirely new cutscenes altogether! However, the downside is that Makou Reactor is perhaps the most difficult tool to master, and that will definitely take some time, so be patient.
Unfortunately there's currently nothing out there that allows you to edit stuff on the world map, as far as I know. As well-documented as FF7 may be, the world map still remains a "black box" for the most part, sadly.
Anyway, the aforementioned tools should get you started on what you want to do. Oh, and welcome to the forum, btw.