To answer this question there are a number of things to think about
When getting a keyboard the things which is most likely most important is the price.
Depending on your price range will affect the quality, size and key action of the keyboard you get.
The best keyboard for beginners below £500 would be either the Yamaha P-45. As these keyboards can last a long time for the beginner user and can be used for preparing for graded exams or for fun.
When choosing a keyboard another important thing to think about is the brand.
As although lesser well known brands of keyboard are cheaper they are also probably lesser quality and you might end up having to get another more expensive keyboard soon. So, choosing good brands for keyboards is another important part
Some of the best well known brands include:
Additionally, for choosing a keyboard thinking about what you are going to use it for is also important. As this will additionally affect the price of the keyboard.
You would need a different type of keyboard if you wanted to plug it into your keyboard rather than do it for exams for example. If you wanted to plug it into your computer you would need a MIDI keyboard. You would also need a different type of keyboard if you needed it to make different sound types.
How big you want your keyboard to be will also be an area which you need to think about before buying a keyboard. If you want a small keyboard then you are looking at a MIDI keyboard. If you want a medium sized keyboard you are looking at 61, 73 or 76 keys. If you are instead looking at a full size keyboard then you are looking at an 88 keys keyboard.
The smallest 88 keys keyboard is the Yamaha P-45.
This is more important the more advanced a pupil is. Key action is whether the keyboard has weighted/semi-weighted/non-weighted keys. A normal piano has weighted keys so some of the better keyboards have weighted keys to make the keyboard feel like a piano.
Non-weighted keys don't have the resistance that pianos and weighted keys keyboard. This makes it more difficult to prepare for exams where the pupil would be playing on a piano for the exam.
Semi-weighted keys have some resistance but not as much as weighted keys keyboard.