Capabilities
Make sure that JLCPCB can manufacture the PCB you want, their capabilities page lists what they can and can't do. At the time I am writing this, they handle up to 6 layer boards. They can't do blind or buried vias (but then again, most generic PCB fab places can't, and I'm hard pressed to think of a design in our applications where such space optimization would be necessary).
Account
I have made a 40m account - contact me or Chub for the login creds. It is linked to the foteee@gmail.com account, but their web interface already allows correspondence with their engineers, who are pretty helpful.
= Common gotchas =
Most times, you would just download the "Fabrication Outputs" zip file from the DCC for the design you want to fabricate. A common problem I have encountered is that often, the board outline is not exported as a Gerber file. This stackexchange details the fix - you have to do this in Altium. Then, you have to re-export the files, following the nice writeup on how to do this.