Shooting legal advice
In Great Britain you can?t buy a shotgun or ammunition until you are 18. You may be given one when you are 15.
There is no lower age limit for being lent a shotgun or for obtaining a shotgun certificate.
Anyone under the age of 15 using a shotgun must be supervised by someone aged at least 21.
As you are a newcomer to shooting I strongly advise that you go to a shooting school and have a few lessons.
The cost will be well repaid in the long run. Shooting schools have a range of guns you can try.
A 20-bore is a good gun for younger people to start with and plenty of adults use them too.
Many over-unders have interchangeable chokes which makes them more versatile.
As to make and model, that is mainly a matter of personal preference and the depth of your pocket. Buy the best quality you can afford.