It really depends on what is the easiest for you and what clinic you want to go to. In BC Canada most clinics would be able to offer HIV/STI testing for example family doctor’s, walk in clinics, youth clinics etc.
Some clinics require you to book an appointment before going, walk in only or a combination of appointment and walk in. I would call the clinic you are planning to visit to see what type of system they have.
Let us know if you need any assistance with finding a clinic in your area. You can also have a look at our clinic finder for a list of some clinics in your area.