Most of the answers for a beginner are are in the Lyman blackpowder handbook, I would get one.
I'm a huge fan of Thompson Center, but unfortunately their older flintlocks are a dud. You can find pictures online of the old vs new design locks. The old ones kinda, sorta work, but they are really frustrating. I would hate to have you experience them and think that's how all flintlocks are. I have not owned the new style lock, I've only heard they are better. You can also buy aftermarket locks for these from L&R, and that's the way I would go if I was to buy another TC flintlock. It might look like a lot, but they would really be worth the save frustration, and maybe even flints. My old style TC Renegade flintlock basically just disintegrated flints. After 10 shots they were in bad shape.
Now assuming you have a good lock setup, here is what I would recommend for a beginners gear. You can buy all this stuff from Tack of the Wolf, which is where I buy most of my stuff. I'll list part numbers
An adjustable powder measure with spout. MAS-120
Rifleman's powder horn. HORN-D
I personally do not like the brass flasks, but they work.
Small funnel for filling the powder horn. FUNNEL_SB
You don't need to buy this, you can find alternatives at home. Even a piece of paper rolled up can work.
12 pack hand knapped flints, 3/4"x7/8". FLINT-ENG-6-PACK-12
Short Starter ST-7
Some load without these, but I would still have one around.
Round balls. I would for sure get .490" and 495". I wouldn't discount .480" either. Get hand cast if you can. Load sprue facing up. Later start casting your own. BALL-490-X, BALL-495-X
Range ramrod. 44" RAMROD-C-44-10
These are expensive, but they are phenomenal. Technically optional though. Don't buy too short, you will probably buy more rifles in the future. 36" would be ok too, you can always buy a second one later if you get a 38"+ barrel rifle.
T handle for ramrod, 10-32" thread. RAMROD-T-10
Loading and cleaning jag, 3/8" rod, 10-32" thread, 50 cal. JS-50-6-10
It's a good idea to have two of these.
Ball puller with brass collar, 10-32 thread. BP-50-10
Depends on your ramrod thread, but all mine are 10-32. The brass collar ones are WAY better.
Lastly, I always make my own ramrod. I've tried all kinds of materials, and nothing has beat hickory. I both epoxy and rivet the ends on. JB weld works great. I can go into more detail if you wish. Technically this is optional, but you either need the range rod, or a good ramrod with threads. You will need the following.
Hickory 3/8" dowel, 48". RAMROD-6H
3/8" Brass ramrod tip, 10/32" thread, quantity 2. RT-6-10-B
Hopefully I didn't miss anything.