I'll tell you what worked for me (a friend suggested it to me after i asked her for advice), though every child is different & will do their own thing when they are ready. I started this around 16 months. I sat my son on the potty every hour on the hour, just to get him used to the routine. If he did anything in it we'd get all excited and be like "yay, look what you did on the potty, you're so clever!", etc. Do that for a few days (how ever many days you feel necessary). Then go from every hour to every 2 hours and so on, gradually, so they are used to the routine & begin to understand what the potty is all about. Also, stretching the time further & further so they begin to know to go them selves when they are ready.
The potty got broken one day (my husband tripped over it), so then i moved our son from the potty, to the toilet (using one of those little kid seats you sit over the toilet seat). Initially he hated it, he literally screamed the house down the first few days. I just held him there and made him sit a few minutes. He got the idea & after a couple days he stopped freaking out & realised that it wasn't a bad thing. It didn't take long till he was taking himself. Just leave the door open & the seat on, and they will start taking themselves. Even if they just sit there, they're getting the idea.
The other thing is, take notice of when your child does his business, whether it's first thing in the morning or last thing at night. That's when you should sit them on the toilet, they're more likely to do something then. Also (sorry if this is confusing, i'm typing while i jog my memory) Only use nappies when you go out & at bed time. If you're home, just let them wear underpants. If they wee or poop, yeah there's a bit of a mess to clean up - but they work out pretty quick that it's gross having a mess in their pants and they tend to start wanting to get to the toilet in time.
Phew. Hope that made sense & was of some use to you.