Just bought a '68 Mustang 289 2v coupe, what should I do first?

[induction]

Thats a really tough question to answer. I think first of all if everything seems to be sound and in good nick just drive your new friend and enjoy it. Then start with the little things she needs to keep her on the road and running smooth, perhaps a carb rebuild , change ALL the fluids( including the rear end fluids)check for things like tatty wireing and electrical problems. At that point you should have gained a bit of knolage about your new best friend and hopefully you will be able to start doing some of the bigger jobs yourself insted of letting someone else have all the fun.
Decide where you want to go with the car like a full restore or resto-mod, or just a weekend racer. You have plenty of time so don't rush into something you wont be happy with later.

JMO
Have fun with your new classic