Welcome
What have you tried?
- create an posts, making comments. Possibly will use a database, maybe CloudDB or FirebaseDB
- displaying posts on wall like on facebook or on forum(depends what's easier to create) Perhaps use Layouts and Labels
- can temporarly set post as "reserved" or "available" This is up to you perhaps Programming Your App to Make Decisions will give you ideas
- save current addres taken from location sensor. Have you read Using the Location Sensor or HOW TO: Parse a LocationSensor's Current Address by SteveJG ?