If you are certain that your billings for the year from all clients will be less than $30k, then you don't need to register for or collect GST. But if there's any chance you will exceed this amount you should register.

This questionnaire is helpful: http://www.cra-arc.gc.ca/tx/bsnss/tpcs/gst-tps/rgstrng/rgstr...

Your next question should be to decide whether to operate as a sole proprietor or to incorporate. Sole proprietor: you will file all your earnings on your personal taxes as usual. If you incorporate you'll have some responsibilities to create and maintain the corporation. You'll likely want to follow the sole proprietor path for a part time gig.

If you do decide to register for GST (or to incorporate) I'd suggest finding an accountant to help you with the setup of your GST account with CRA and to explain the schedule for remittance. It's actually not hard at all but helpful to understand from the start.