Start by buying a potty seat and take your child to the bathroom every time you go. I don't expect an 18 month old to "use" the potty but they will start talking about the potty. I call it "suggesting" potty training.
Read stories about the potty at this age. "Elmo goes to the potty", " Dora has a potty", anything that your child enjoys reading that discusses what we all do in the bathroom. You want them to understand that everyone goes to the potty!!
Once your child is showing signs of understanding going to the potty and is actually performing when you take them (not just sitting), it is time to get even more serious about potty training. Get out the star and sticker charts and I used a candy jar with M&M's as well. Start rewarding each success!! The more whoops, hollers and claps, the better.
After you know they are "getting it" real training starts which means they need to be wearing training pants. No pull-ups because children don't mind having an accident in a pull up. Then, take your child to the bathroom every couple of hours. Don't ask "do you need to potty?" Tell your child "It is time to potty!!". Make sure to reward each success!
For most children, it will take a few days to weeks before they are actually more trustworthy and they will let you know they need to go and you don't have to take them quite so often.
Once your child is interested start potty train them! If you miss the window of interest, it may be weeks or months for that window to re-appear and you do not want to miss out.
I'm Dr. Sue with The Kid's Doctor helping parents take charge.