I am a new cook, and I want to freeze roast beef for later use. So I cut it in 2cm pieces. I never cooked roast beef before and realized that it is cooked as a whole.
What can I do now? Should I season it like steak and cook it like steak?
You are not giving very much information to provide a quality answer. From a safety point of view, yes, you can absolutely cook them like steaks.
From a quality and palatability point of view, that may not be ideal, depending on where the roast from which the steak were cut came from on the animal.
The question is, are they better suited to rapid, high heat cooking (grilling, pan frying, and so on) or low and slow technique (such as braising)? Observe the cut steaks: are they full of connective tissue, and interspersed fat, which appear white or pale? If so, you probably (unless you have a very expensive piece of meat like prime, which you have not mentioned) have something suited to low and slow. If they are fairly uniform in red, meaty color, they are probably suited to grilling or pan frying.
For low and slow: I would suggest a braising technique, as for pot roast.
For rapid: I would suggest grilling them or pan frying them at high heat. Cut them across the grain if you can see and identify it.
This is not a very detailed or satisfying answer, because you have not given enough information on what you have, and roasts can come from all parts of the animal, and yield steaks with very different characteristics.