In order to start a plant you have to have a seed. (I’m ignoring cloning plants because you still need to have an established plant before you can try that.)
Cannabis seeds are legal to own in most parts of the world, but in many places they are also illegal to germinate. Yeah, I know: weird dichotomy, right? If you need this stuff for your health, as I did and do, don’t hold back – but do be appropriately careful about how you grow, where you grow, and who you tell. Normal advice on that last point is ‘tell nobody at all,’ but that’s kinda a theoretical position. Just tell as few people as possible. Even if it’s legal in your state or country, an outdoor grow could be targeted by someone keen on grabbing your harvest before you do.
Anyway, there are MANY different cannabis seeds vendors around the world, and many of them will mail your order internationally as well. Some will also optionally mail your seeds in disguised packaging: in DVD cases, with candy, a t-shirt – anything that provides a bit of misdirection. That can be overkill, but if you’re concerned it can certainly give you some peace of mind.
I have used Seedsman regularly for a few years now and it’s always been great. I definitely recommend putting that store at the top of your list. I’ll add more as I find ones that fill me with confidence, but please let me know in comments below if you have ones you would recommend and I’ll consider adding them too.
Once you have your seeds you’ll need to germinate them, so go read How to Germinate Cannabis Seeds for my advice on that. Hint: do not use paper towels!