The price you pay for your domain can vary greatly between registrars. When comparing prices, take into account the other services included by the registrar, their customer support, testimonies, etc.
Your chosen registrar should be ICANN accredited. ICANN maintains a list of accredited domain registrars. If the domain you have chosen is a reseller of an accredited registrar, they should let you know which registrar they represent.
Take the time to read and understand your domain registration terms. Pay attention to ownership clause and other terms of condition.
Do not register your domain for more than one year in the beginning. That give you time to evaluation your registrar before further commitment, as well, allowing you time to evaluate your domain name in case you wish to use a different domain name.
A domain name registration generally takes no more than 48 hours.
You should be able to transfer your domain to another registrar without any restriction other than those enforced by ICANN. Check the registrar for other restrictions and curbs.
Check if phone, chat or email support is available.
Does the registrar provide a domain name management panel for you to manage your own domain name? Or do all changes have to go through them?