You can start Professions starting at Level 10. You can easily access your professions page by pressing the N key on your keyboard. Alternatively, you can press the Alt key on your keyboard then click the middle button on the toolbar at the top of your screen (labeled N, it’ll look like a hammer and chisel).
This is what the Professions page will look like:
While it is completely up to you to decide which profession to start in, I recommend Leadership. Leadership rewards free experience points, ardent diamonds, gold and much more.
You can also do a short Professions Quest given by Sergeant Knox in Protector’s Enclave. Available at Level 10, just talk to Sergeant Knox and accept the “A True Professional” quest.
He’ll task you with seeking out M’Tari Prin’tal, the Profession Supplies merchant, who is located in Protector’s Enclave within the Seven Suns Coster Market. Talk to her to complete your quest!