I own some stocks, but i have no idea how they work.
Any idea of how to use or sell stocks?

Stocks can't really be used; the only thing you can do with them outside of trading is using them to view the status of that faction.

To buy or sell stocks, there are a few places this can be done:

1. The Galactic Market: www.swc-gm.com (basically SWC's version of eBay)

2. The RPG Centre -> Commerce or Auction House, depending on if you want to sell or auction it off.

3. The #swc-traders IRC channel, if you want someone to purchase it real-time.

AND..
Always remember to use a good middleman when trading with a person you don't trust. Two resources which list most good traders and thieves are:

1. Enigma's Traders Reputation List - http://members.chello.nl/~o.koek/traderslist.htm

2. Traders Protection Agency (TPA) - http://pub238.ezboard.com/btradersprotectionagency

or you could just wait untill the NPC market comes out
"points towards syn and crew"
ahhhh they are after me!!!

NPC market is not currently being implemented.

How do you know how much your stocks are worth?
And what would be the estimated time the NPC market would open?

Best way to determine the worth of stocks would be to visit the GM and check what others are currently selling them for, then pick a value somewhere in the middle. (:

No known ETA for an NPC market.. because it isn't currently being worked on. So you'll have to live without it.