Whether to use a potty or go straight to using the toilet depends on the parent and child. Many parents will tell you the potty is less intimidating because there's no fear of falling in. Also, the potty allows children to keep their feet on the ground which can help them push down during a bowel movement.
If you decide to use the toilet, you will want to buy the seat that attaches to the toilet which is better for smaller bums.
Here are just a couple of examples:
You will also want to buy a small stool so they can plant their feet and feel secure and stable. It will also help them get on and off of the toilet. No need to spend a lot of money on them. A simple stool like this one would work well.
If you go with a training potty, there are many options. Some will have your child's favourite characters, others will play a song or cheer when the child goes in the potty. Not all potties are created the same so here are a few things to keep in mind:
For a comparison of some popular potties and toilet seats, see this link from Parents.com.
Some parents choose to buy a potty or seat for every bathroom in the house so it's easy to access when they need to go. If you're taking a road trip, take the potty seat with you and stop every so often to give them a chance to go because you might not find a restroom when you need one.
You could also buy a portable potty which can easily be carried wherever you go so your child can use it on the road.