If you dont have a bike, you need one. If you do, you need four pegs, and it will escalate from there, trust me.
Other flat-specific parts that I recommend to get first:
zero forks
short stem
They will change the feel of basic tricks such as forkglides, hang-5s, and steam rollers more than anything. (the headtube angle would be included in this group, if it were a part lol)
If you have the cash, get a complete opsis for ~$300. If you have more, get something better, or start with the opsis and upgrade.
I personally think forkglides->flatland like bunnyhops->street, so I say learn forkglides first.
Oh and BUY GROUND RULES. Amazing DVD