As everyone else has said, step one is CBT. Well, or step two, if you don’t hold a full driving licence or otherwise have provisional motorcycle entitlement.
CBT is Compulsory Basic Training and is exactly what it sounds like. It’s a day’s course, give or take, and will teach you skills such as “stop,” “go,” and “turn without dying.”
From there you can either learn restricted (125s) or take a DAS course. There’s pros and cons to both.
Restricted is cheaper. When I was learning, DAS lessons were dearer, and the only experience you get is with an instructor, as opposed to getting a 125 and hacking about on L-plates on your own. There’s nothing to stop you doing that anyway of course, but it’s probably pointless unless you plan on taking DAS and then keeping riding a 125 afterwards.
DAS gives you unrestricted choice once passed, as slimjim78 says.
With DAS you learn from day one (well, day two) on typically a 500cc bike, so you don’t have to learn a 125 and then later re-learn something else. Also means you get all the testing out of the way.
Restricted makes sense when you’re a teenager but to be honest, unless you’re strapped for cash I can see little compelling reason not to just go DAS and then the world’s your mollusc.