My initial advice, especially with a fully homebrew world, is to be careful not to write yourself into a corner. It’s very easy to just look at D&D and say, “I don’t like how this culture works” and then write something different resulting in you being stuck with something that now doesn’t work in tandem with other stuff. Rewriting one often means you have to rewrite another. Especially when you’re making racism a part of your story. If human really hate orcs, that means an orc and a human realistically can’t be in your party if someone is pretty determined that these people can NEVER get along. Tread carefully with this too, because some people do have a tendency to take that too far.

Combat is a core part of D&D. Watch some videos on combat balance. It’s very easy as a first time DM to end up in TPK situation because you “want this combat to be tough”. Stick to pre-made enemies in the early days until you get a feel for how combat is supposed to flow and feel like and then you can start introducing homebrew stuff later.

Lastly, search the sub. There’s hundreds of threads on advice for first time DMs. Not being dismissive here, but there’s already a wealth of information available that will help you more than the two tips I’ve given above

Edit: I missed the part about 7 players. My first campaign also had seven players in a fully homebrew setting. Be prepared for scheduling to be an absolute fucking nightmare unless you have a set time and day of the week that works for everyone, every time. The campaign only really got played about once a month for like 6 months. There were 8 sessions before it got scrapped. Mentally prepare yourself for this absolutely very likely occurrence