Welcome to PokéCommunity!
Well, the first thing you need to create a ROM Hack is a ROM, of course. I cannot link you a ROM, so you'll have to search it for yourself. However, I can advise you to which Pokémon game you should choose to create your ROM Hack. Since you're new to ROM Hacking, a good call is to choose a game that you'll find a lot of resources to work with your ROM. In this case, you should choose Pokémon Emerald or Pokémon FireRed (beware of FR's version: you have to find the 1.0 ROM, as the 1.1 version won't work with most tools and resources).
After that, you're gonna need some tools to edit your game. They are a helpful hand to edit simple things in the game: pokémon stats/type/moveset/evolutons, moves BP/accuracy/effects, pokémon encounters, and many other things. You'll find a lot of these tools in this thread:
https://www.pokecommunity.com/threads/404159
Tools that worth a mention that are in that thread are Advance Map and XSE. The former will help you to edit maps (of course), and pokémon encounters as well. The second will be necessary when you begin to work with scripts.
Another useful tool (which is not mentioned on that thread) is PGE (Pokémon Game Editor). It will help you to edit a lot of things - pokémons, moves, TM/HMs and many other things. You can find PGE here:
https://github.com/Gamer2020/PokemonGameEditor/releases/tag/3.8
If you don't like PGE, you can also use G3T (listed on that thread) or G3HS (here:
https://www.pokecommunity.com/threads/320113) - but really, you should go for PGE.
With these tools, you can change most of the basic things of a game. However, there are things that cannot be easily changed with these tools: you're gonna need to use scripting, assembly routines and hexadecimal editting. You should check the following threads:
General changes (i.e: making Mew obey in FR):
https://www.pokecommunity.com/threads/205158
ASM Routines:
https://www.pokecommunity.com/threads/339153
Moves (updated effects, Gen 4~6 moves and stuff):
https://www.pokecommunity.com/threads/300524
Abilities (new abilities, updated effects...):
https://www.pokecommunity.com/threads/343264
You're might want to know how to insert ASM routines into the game. Check this thread:
https://www.pokecommunity.com/threads/313906
Well I'm not an experienced ROM hacker, actually I struggle to do a lot of things, but some basic things I might help. Feel free to ask, and make some search on Research & Develpment subforum (
https://www.pokecommunity.com/forumdisplay.php?fn=research-development) and Tools, Tutorials & Resources subforum (
https://www.pokecommunity.com/forumdisplay.php?fn=tools-tutorials-resources), there are plenty of things you can find there to help with your ROM Hack.
Good luck and happy ROM hacking!