You learn how to do this yourself, basically. Flick through the a/h listings (in the starter cities, where you probably are, it's only really worth checking the prices of things that are for sale already. Most things will have been listed a long time ago but don't sell.)
If you're in Windurst, Silk Thread goes well.
If you're in Bastok, Beehive Chips and Ores.
If you're in San d'Oria, Sheepskins, Bat Wings and Flint Stones.
As for partying, there's a party section in the menu so you can turn your party flag on/leave a party.
It's easiest to search for members via /sea (game command for search)
/sea all = search entire world
/sea "East Ronfaure" = searches players in East Ronfaure
/sea all 15 = searches all players level 15
/sea all 15-20 = searches all players between levels 15-20
/sea all whm = searches all White Mages
/sea all Likibiki = searches for someone called Likibiki
Note that you can only invite someone in the same region as you. The invite box will be greyed out if they're in a different area.
Also something to note is that almost all players find it polite to send them a /tell first, before inviting them to party. I know I prefer it. "Blind invites" (as they're called) just feel slightly rude, it's nice to get a quick "Hi, would you like to party with me/us", then wait for a reply.
Hope this helps, best of luck!
Likibiki ~ 75 WHM ~ 75 SAM ~ 75 THF ~ 75 SMN ~ 74 NIN Pandemonium Server, FFXI. Retired. Gone but not forgotten.
Cailey ~ 90 DNC ~ 90 RDM ~ 90 BLM Asura Server, FFXI. Adventurer Extraordinaire.