Building a raised bed yourself can be an interesting project and allows you to create a highly individualised bed. However, building your own requires a certain amount of craftsmanship, strength and, last but not least, patience and time.
If you decide to build your own raised bed, take a look at this video, for example:
On the other hand, a bought raised bed is often more expensive and you are limited by the given dimensions.
A good compromise can be a ready-made kit. This has the advantage that it can also be assembled by a beginner and is often cheaper than a ready-made raised bed.