I was an instructor and ATB (bike school) owner between 2004 and 2014. I taught: CBT, A1, A2, DAS for category A, ERS - DVSA post test / advanced riding, IAM, Rospa, and occasionally on trackdays at Snetterton, Brands and Silverstone.
In 2014 I turned to the dark side and became a DVSA Examiner - meaning i went from poacher to gamekeeper. I changed career in August 22 and completely moved away from the motorcycle industry, after being a professional motorcyclist in one way or another since the mid-90's, but I think most of my knowledge is still relevant.
In the UK: The process is the same for all of the full licences.
If you want to 'dip your toe' get yourself a CBT and razz around on a 125cc for a few months and see if you get on with it. It will be your cheapest option...but make sure you ride. Make excuses to go for a ride, not excuses why you can't!
Without splitting hairs, obtaining a full licence will be a similar cost no matter which category you take, unless you provide your own bike. In which case it's makes more financial sense to go straight in and do DAS (big bike) - if you can handle the weight and size of the bike. After you pass you can ride whatever you want.
If you want to understand a bit more about the process of getting a licence then below is a repost of something I put on the 'Show us your motorbike ' thread a few months back.
There are three tests and a CBT.
The process is the same for all 3 licence categories: A1, A2 and A.
The venue is the important factor:
At a Training School, ATB, Bike School, etc: it's training.
At a Driving or Theory Test Centre it's a test.
To obtain a UK licence you need:
A CBT - which is Compulsory Basic Training. Often mistakenly called a CBT Test. It's NOT a test. It's a training course normally completed in a day, but can take longer if the required standard of riding is not reached. A certificate is issued on completion by a DVSA approved Instructor at an Approved Training Body. A CBT is valid for two years, after which it will need to be taken again if all three of the following tests are not completed successfully. The CBT can be renewed if it expires during the rest of the rider's development and has a separate 'life' to the other documents.
Theory Test: Taken at a Theory Test Centre. Computer based combination of multiple choice questions and Hazard Perception CGI video clips. Some might argue there are two tests - Theory and Hazard Perception - but they can't be taken separately, so it's a two part single event test. There is a target score that must be achieved for both parts to pass. You may be required to complete a theory test prior to CBT if you are only in possession of a provisional licence. On passing the theory test you will then be allowed two years to complete the following two tests.
Module One Test: a machine control assessment conducted by a DVSA Examiner on a Motorcycle Manoeuvring Area (MMA) at a Multi-purpose Driving Test Centre. Successful completion of this test allows the candidate to progress to the final test. The life of the Module One pass certificate is governed by the life of the Theory Test pass. They expire on the same day. Module Two must be completed before they expire otherwise the candidate has to resit both tests. CBT runs on its own timeline.
Module Two Test: essentially a combination of all three of the above.This test is conducted on the road with a DVSA Examiner following on a bike, or sometimes in a car, giving radio instructions. Again this will be conducted from a Driving Test Centre.
If you're a complete beginner it can be challenging / hard. Most likely to come unstuck through a lack of machine familiarity / control and / or poor Highway Code knowledge.
An experienced rider shouldn't find it particularly difficult. Control is not normally a problem but Highway Code knowledge often is.
Most riders mistake experience for the time documents are held. E.g. 'I've had my CBT for 6 months'. This statement is meaningless. As is: ' I've got lots of experience as a pillion'.
Hours riding / miles ridden = experience. Nothing else. Variety is key. Repeating the same monotonous short journey doesn't count.
Sitting on the back of a bike ridden by someone else gives you no more experience than being a passenger on a plane gives you experience of flying it.
The cheapest resource available is the one thing that 99% of candidates will avoid and most likely will fail for - The Highway Code* - and yet they'll complain how expensive it is to take another test.
* As an app the Highway Code is free and yet so many feel they don't need it: 'but I've done my theory....'.
Quora post over...
If there's anything else you want to know just ask.
The best advice is that which you don't take!