There are three main ways to obtain a domain name: register it if it is available, buy it from the current owner, or try to catch it through backorder if it is expiring.
A new registration is the simplest option. You choose an available name, register it through a registrar and renew it every year or for a longer period.
A purchase is needed when the domain is already owned by someone else. In this case the price depends on the owner's expectations, the quality of the name, traffic, history and market demand.
A backorder is used when a registered domain is close to deletion. It can be cost-effective, but success is uncertain because the current holder may renew it and other buyers may compete for it.
The right option depends on urgency, budget and how strategic the exact domain is for your project.