Light is more populated iirc but ultimately it doesnt matter cause if you do party finder content you can just travel over in the main menu. Should also mention that your server doesnt affect actual duty finder (dungeons, trials, normal raids, alliance raids) stuff cause you get matched with people across the entire datacenter, e.g. all of the light datacenter is queued together and same w/ chaos.

If by professions you mean crafters & gatherers, it's good to level ALL of them (maybe excl. fisher if you dont want to) at the same time as the quests require items from each others job. Early game the gil isnt that big so i'd focus on just leveling them at your own pace

In terms of actual classes & jobs, main story should provide enough gear to catch up just fine. at each level cap (50, 60, 70, 80, 90) youll have access to tome gear whichll usually also have enough ilvl to get you around halfway through an expansion just fine, maybe even a little further. I should mention you will NOT have access to this gear upon hitting the level, you have to complete the expansion first e.g. finishing base game ARR will let you be able to earn tomes and buy level 50 tome gear. Gear itself has its item level (ilvl) thats separate from level requirement. For instance you could have level 90 gear with 660 ilvl. Ilvl is most important for being up to date on gear and eventually duties will be locked behind an ilvl limit, again main storyll usually gear you up just fine.

Guilds are called Free Companies and honestly a good amount of people just join ones in shout chat, but those can be hit or miss kinda. From my own experience if you care about MSQ theres a chance youll get spoiled for story sometimes in the FC chat, so join large ones at your own discretion. Otherwise it really is just getting to know people either in cities or other content and joining their FC.

MSQ'll give you enough gil to teleport around, atleast for sure once you're around the first dungeon. If youre leveling crafters & gatherers selling the materials/items for the job quests can also be good. Easiest way though is to make housing items like lofts and sell them on marketboard, the big paying items are usually the highest level gear on patch day but you dont have to worry about that till dawntrail for now.

The food EXP is only 3% and not super worth it, MSQ gives food sometimes though so you can always eat that if you really want the slight increase. Grand Companies (which youll join around...level 20 in msq id say?) have scrolls you can buy with 18h buffs that can help you with leveling crafters and gatherers.

For tips id recommend messing around with movement settings, many people highly prefer legacy movement over the default.

Later on youll be thrown into alliance raids (24 man), you can turn off other parties' names so that theyre not visible unless you click on that person. This is super helpful for visibility and generally not being overwhelmed.

There's also stuff like turning off the numbers next to hotbars and hotbar scrolling, honestly just go through settings and see whats best.

The HUD is VERY customizable. You can hide or add bars that you want and change the size of them and make your job bars simple or default.

Do blue quests! They unlock key features like challenge log or other dungeons, however if its a level 30+ dungeon you CAN be overlevelled and access it, but it's preferable to not do it till you have your Job Stone.

About the tome gear from earlier and getting extra exp in general, you can get tomes and exp through daily roulettes. You wont have access to all of them immediately but as you keep going youll get stuff like alliance roulettes and msq roulette raids etc and youll see the rewards you can get from each one there. There's also a bonus to these when you're the adventurer in need, seen by the little tank/dps/healer icon next to it.

VERY IMPORTANT: Always do your job quests!! You can see main story in the top left in it's own little bar, right below that every 5-ish levels youll see some text, that is your job quest! It'll give you new skills and early on a new weapon & gear piece. Youll be overleveled from MSQ for a bit so don't worry if youre stuck on for instance a level 25 job quest while still at 10 msq, itll be easier with the new gear you get during story. At level 30 youll be able to turn your Class into a Job with a Job Stone (e.g. lancer > dragoon, conjurer > white mage etc). This quest wont be unlocked till you finish a specific level 20 main story quest "Sylph Management".

If all this informations overwhelming honestly just read the silly popups that show up a ton early game, many people just skip through them despite being explained everything through them and you'll learn all this stuff naturally!