Need Clarification

[Amaterasu]

Getting ready to go out of the Nexus for the first time. How do I do the posts? Do I wait for someone to reply, or simply keep replying to my own posts? And when I get to an area like one of the towns, what then? Am I supposed to attempt to do 5the town's reaction to my character, or is that something a mod or someone else will do? Also for fluff encounters like meeting someone on the road, I take it that it is ok to simply play them? Little bit confused, hoping for clarification

Presuming that you are not waiting on anyone you can just go ahead and write into whatever verse you want to go to, be it in a new thread or joining an existing one.
As for towns and the like, you are mostly free to go about their "reactions" to you yourself. You can request aid from the staff if you are in a pickle or just need some aid, but it's not a requirement (if you do however, use the Cry for help thread). If you want to play as a site NPC just remember to request permission for that first. Same thing for meeting someone on the road.
I'd recommend reading the "What to do once you've been approved" thread and possibly the rules for more info by the way. There's a few great resources that'll help you get situated