If you decide to buy a VPN, to make sure that you get a good VPN, you need to have at least this features
Good support. Problems or questions will always appear, not necessarily because of the VPN provider, but when you need help, a good provider must be there for you and help you fast.
Strong Encryption. You probably need a VPN for privacy or to pass some restrictions, for both, you need encryption. A good VPN will have strong encryption protocols like OpenVPN P, IKEV2, PPTP, etc and will not have the DNS leak problem
Multiple VPN locations. A good VPN will have a big list of locations so you pick what is closer/faster for you.
Multiple devices. You will probably want to use the VPN on your computer, tablet/phone at the same time. Pick a VPN provider which allows multiple devices.