Potty training

My son turns two next month and I was thinking about going ahead and starting potty training. I've got a little potty that sits on the floor and I also have one of those seats that attach to a normal toilet and I'm not sure which to use. Also I already have him in pull ups but I was wondering if I should go ahead and start putting him in underwear, and do I just need to teach him how to pee sitting down for now? I'm just not sure what to do, I'm a single first time mom so this is new to me and I'm not sure where to start.