In short
A true reservation of an already-taken domain generally does not exist: either a domain is free and gets registered, or it is taken. For taken domains that might soon become free, a backorder is the right option — an advance order on the re-release, but not a purchase guarantee.
Free domain: register immediately
If your desired name is free, it is registered directly — an advance reservation is neither necessary nor common. See registering a domain.
Taken domain: backorder instead of reservation
If the domain is taken, you cannot "reserve" it. If it is likely to become free, a backorder is an option. Whether and when a takeover is possible depends on the deletion cycle of the respective TLD.
Realistic expectations
A backorder improves the odds but guarantees nothing. For popular names there are usually several interested parties.