How to write/visit someone in prison?

I see from other questions you asked that this is a British question (and an American wouldn't mention crisps!), so this is something I know about!

Assuming you know the prisoner, even then you need to know their prison number or your letter won't get there. You can't just write to anyone. HM Prison Service works entirely on numbers - I've known a fellow prisoner, my cellmate in fact, who got a letter from a solicitor that certainly wasn't his, simply because it had his number on it. Wrong name, wrong prison, from a solicitor he didn't know, only the number was his. Clearly a typo by the solicitor's secretary.

To visit, it has to be set up in advance so the prison can approve the visitor - prisoners are prevented from visits by, or contact with, anyone who isn't approved, such as victims of their crime. This is a particular issue for sex offenders, as the most common type of sex offender is men who abused their own children. Should they be allowed to phone home?

But you don't know anyone and just want to correspond with a prisoner... the only way of doing that is to google a prison pen pal scheme (these are mostly run by religious charities), and that will list prisoners who want a pen pal. Being under 18 may well prevent it. But then you have someone who WANTS to have someone to write to, and I tell you, especially for a long-term prisoner who doesn't have anyone to be in touch with, getting a letter can be the highlight of the month! But even I, as an ex-prisoner, will say be careful with revealing too much about yourself. For all you know, your pen pal might be a psychopath who just wants to play mental games with you. Most aren't but you can't know that. Get to know them and then a visit might be possible.

You are a minor, you aren't going to be allowed anywhere near a prisoner unless they are a blood relative and you have your parent's permission. Criminals aren't monkeys in a zoo, you can't expect to leer at them for amusement. If you are interested in criminology, you will find that it requires a lot more empathy and respect for these people than you are showing right now.

I am not sure but I think that the rules want minors to have permission from a legal adult to visit someone in custosy

There are prisoner pen pal programs. You can talk to your parents about looking into them. You (and your parents) might also consider contacting a local parole/probation officer for more information.

You will have to go in with a parent or guardian. then sign some papers to make sure that you actually know the person. they will then screen you and check your background. when you want to see the person, you will be searched and they will look you up and do to make sure that you don't have any weapons or lock picks or anything of that nature. You will then be allowed to visit that person.

There are just prison pen pal programs. But I would not do this as I would not want to associate with criminals whatsoever.