When you start potty training it is important to remove diapers because they are like a crutch for children. The comfort they have in diapers allows them to pee or poo in their diaper rather than in the potty.
Decide whether you want to start with training pants/Pull-ups or go straight to underwear. The latter will be messy if your child is potty-training outside of the home but many parents find going cold turkey to be most effective.
If you choose to go with training pants, it can be beneficial to let your child play in underwear or naked for a few hours every day when at home. This will help them to become more aware of when they need to go without the safety of a diaper to catch it all. You will likely have some messes to clean up but the wet feeling may be what it takes to encourage your child to use the potty.
If they have an accident, it's important not to get upset with your child. Instead, remind them to use the potty next time or to tell you when they need to go. If you lose your patience with them, they may get scared and hold it next time.