how do i start potty training a 2 yr old

Start out slow. Get them each a potty chair that they get to pick out. (This will make it a little more fun for them.) At first it helps to have the potty chairs in the bathroom you use most so that you can take them every time you go to the bathroom. You can start by having them sit on the potty chair with their clothes on, pants down but diaper on, or diaper off, whichevern they are comfortable with.

As they get used to the process they may start peeing while you're going to the bathroom, and that's great! Once they understand what the potty chair is for you can take them to the bathroom every half hour or so. The more often you take them the more likely they will be to use the potty instead of their diaper.

You can switch them to disposable training pants once they seem to be understand potty training. Once they're using the potty chair reliably you can go to cotton training pants or regular underpants. 

Just know that potty training can be a slow process. One thing that worked with my daughter was (after she understood the idea of potty training) putting the potty chair in the living room so that she could use it whenever she needed to. She liked being in control of the process, she got tired of me taking her and controlling everything. But, that only lasted until my son started crawling and playing with her potty chair.

Rewards can also help. Kids like getting stickers or small pieces of candy (a single m&m or Skittle) for each successful trip to the potty.