For missions go to your main menu and click profile, you should see a progress bar, some missions only open up when that bar is three-quarters full or what not as well as possibly having pre-requisite mission requirements. To fill up that bar you'll either need to redo an earlier mission or you can trade stacks of crystals (of any type) to the NPCs that also grant you Signet.

As for meeting ppl, lvl 15 is a decent level to start heading towards Gusgen Mines where a majority of low lvl characters congregate to spam Grounds of Valor you might be able to find a linkshell there.

To head to Gusgen Mines from Windurst is a bit of a long trek but pretty doable at your level just be sure to avoid Goblins Yagudos and Quadavs. Also buy an Instant Reraise scroll (from the NPC that grants you Signet) and use it then buy another one immediately before you start your journey.

As far as zoning is concerned, should go something like East Sarutaburata head north towards Tahrongi Canyon > Tahrongi Canyon head due East (path twists and turns then huge open area) towards Buburimu Peninsula > Buburimu Peninsula head due south following the road to Mhaura a Port village take the boat at Mhaura to Selbina (be sure to stay below deck if the music doesn't sound at all like a boat ride monsters can spawn) once in Selbina you'll be at the doorstep of Valkrum dunes > Valkrum Dunes: head East then Southeast through a tunnel and you should arrive at Konstacht Highlands from here head Southeast avioding Rams Quadav and possibly undead and you should see this giant structure called the Crag of Dem, go to it and examine the crystal telepoint (there's a shattered one and a normal one examine the normal one) to get a Dem Crystal.

This allows White Mages with the Teleport-Dem spell to instantly teleport you back there at the crag. After that head pretty much due East of the Crag to find Gusgen Mines. Once in Gusgen mines turn on your party flag and using the shout command (/sh) type in the word Any hit the tab key and select the phrase "Any Vacancies?" from the drop down list hit space and type in the word please hit Tab again and select "Please Invite Me" what you used was the Auto translate function and although limited any phrases you enter utilizing will be automatically translated on the other end of another player's client should their client be in a language other than English.

Its not a perfect translation mind you but it gets the message across. Once you get an Invite ask what page and they should give you an answer, once they tell you the page # examine the Grounds Tome near the entrance and select the training regimen associated w/ the page # they gave you.

Next be very careful and select Field Support and select Circumspection for 5 tabs to get sneak and invisible; you have to be careful cuz it's sandwiched between 2 teleport options and you don't wanna hit those after coming so far! once you are stealth-ed meet up w/ your party and hopefully you can make some friends and join a linkshell which is a group of players united by common interest and chat channel. Best of luck ^^b

Also it would behoove you to familiarize yourself w/ the website FFXIclopedia it is a database site there is another called BGWiki but that pertains more towards Endgame Events as some of their old-game tabs are not as expanded upon quite as much as FFXIclopedia. For getting to Gusgen mines I suggest throwing in to the search box the Names of the Zones and pulling up the maps of each zone so that you get sort of a visual as to how navigate the zones.

Also apologies for making it sound so complicated and daunting but hey it'll give you something to talk about once you get to Gusgen mines and maybe be a conversation starter ^^;

And if you're lucky you might get a bite on this forum post and have someone willing to extend an invitation to a linkshell in Windurst itself and actually guide you to Gusgen. Quite a bit of the game however will rest upon your own initiative so if need be buck up and start exploring the lands between Windurst and Gusgen ^^

Happy trails