I have a problem.

I bought NightFall today and found out I couldn't play an Assassin in NightFall from the start, so I just started in Factions.
I was about to change class when I found out I couldn't have Dervish as my second class, why is this? I have Nightfall.

Whats the problem?

if you want a A/D, you must first choose a different secondary, progress the factions storyline until you can enter elona with your assassin, then progress in that storyline until you can switch to a different secondary, where dervish will be available.

you can also progress until you've reached senji's corner in the factions storyline, then go to the great temple of balthazar (through the isle of the nameless) and switch to dervish secondary there.

Whats a good secondary to get me through the Factions campain.

it hardly matters, but i'd recommend warrior for the kurzick/luxon pve only skill [["save yourselves!"], which effectively makes your party invulnerable for a few seconds at a time.

if I read your post correctly

your trying to make a A/D
the reason you can't switch in factions is because dervishs weren't made in factions. you have to go to Nightfall, Sunspear Camp specifically, to change your assassin to dervish.

There is a quest you get in Kaineng center that will let you travel to nightfall, from there you gotta do consulate docks to get to the mainland and do several quests and missions to get to the camp

step 1. get quest to travel to Nightfall
step 2. beat consulate dock mission to get to mainland
step 3. travel round and get to Sunspear Camp where you can make a A/D

I thought because I had NightFall I would be able to change my second profession to Dervish in Faction, I'll just go A/W for the moment then, thanks.

You can change your 2nd profession to any of the 10 classes in any of the games + Great Temple. But you cannot create a character initially with the second class from nightfall. You need to become Wen Su Noh in factions by beating the Vizunah Square mission, then talk to the profession changer npc in Senji's Corner(factions) Or you can go to the Great Temple of Balthazar after becoming ascended or Wen Su Noh to change, no need to travel to Nightfall and progress there if you don't want to.

Best way is to get ascended and switch in Great Temple of Blathazar...