Hi, I'll try and answer as best I can but if you have more questions probably best to ask in the Battle for Azeroth thread on here, if you use Reddit then the main WoW sub also usually has plenty of helpful people on. You might also see me or others from the WoW thread online in the OCUK discord (check the sticky thread).
For race and class all I would say is focus on what appeals to you and what you will find fun rather than what's 'best'. Yes each race has different 'racial' abilities or traits but in the grand scheme of things the difference in endgame performance isn't massive and unless you're trying to get world or server first kills on new raid content they don't ultimately matter. There will always be a 'best' tanking class or best dps class at any given time but this will also change with time as patches go by so really it's just best to play what you enjoy rather than always chasing the current flavour of the month, and again unless you're doing high-end raiding it won't ultimately matter that much. So pick what you like the look of, maybe try out a few different classes to find the one that appeals most, if you had a favorite class in DAoC (I didn't play) then maybe read descriptions of the WoW classes to see which is closest to that.
Leveling - as this is your first time I would just say take your time, I know it's tempting to just rush to endgame, or buy a boost, but personally I think it's better to just play the game, learn as you go, make mistakes etc. You can get xp from all activities so quests, battlegrounds (PvP) or dungeons - I think last I looked into it questing was the 'best' or quickest but this may have changed. In the past each zone was set to a certain level and you'd do them in certain order, now they've changed this so within an expansion's set of zones they all scale to your level so you can essentially follow your own path and enemies should always be around your level. You'll find mission boards in cities and inns which will give you quests pointing you to different zones.
There is PvP in several forms the main two you'll be able to do early on are battlegrounds and world pvp. Battlegrounds which are instanced PvP battles between teams from each faction, you can start queuing for these at level 20 I think via the group finder (a button will appear amongst the others in the bottom right, looks like a green eyeball). When you're out in the world questing you can also choose to enable 'War Mode', to do this you need to visit Stormwind (if alliance) or Orgrimmar (if Horde) and open your talent window, on the right hand side you can enable war mode - you'll now be phased into zones with people who have also enabled war mode meaning people from the other faction can attack you and vice versa. You get some extra xp with war mode enabled but this is meant to offset the time you'll spend fighting (and dying) due to random PvP encounters which will very rarely be fair fights but some people enjoy that. You don't lose anything when dying in PvP other than the time running back to your corpse so don't worry. If it isn't for you and you just want to relax and quest you can go back to Stormwind or Orgrimmar (depending on your faction) and turn it off.
The UI by default you can move some but not all elements around the screen, for example the character and target windows you can right click to unlock and move but the action bars or mini map you can't. You can however download addons which allow you move everything and customize the UI as much or as little as you like but maybe just get used to the game itself first. When you do decide to look into this one site would be:
https://wow.curseforge.com/addons I believe an addon called 'MoveAnything' would probably do what you're looking for but I've not used it myself. There's a guide on installing addons manually here on another site that hosts addons:
http://www.wowinterface.com/forums/faq.php?faq=install
Lastly I'd say if you decide to stick with the game try and find a decent guild to join, you'll find some guilds will just invite anyone and everyone randomly, personally I'd avoid those and maybe look for a guild who in addition to raiders or pvp players also recruit what are usually called 'social' or 'casual' members, if they have an active group of people you can talk with that will help while leveling and when you do reach max level hopefully you then have a group and some friends with whom you can explore that.
or Horde Goblin yeah tanks and healers will get you a raid / group asap damage dealers have to wait longer if your using looking for raid or looking for group. healing classes priest the easiest IMO Druid, shaman, Paladin i found the druid hardest to master.