Pick up a pencil....

It depends in part whether it is an actual letter or an email. Letters normally start with Dear... . If you know the name of the person, they normally finish 'Yours sincerely'. If you have started Sir/Madam, then you would finish with Yours faithfully'.

There are many types of letters. Can you give extra details?

In a formal context, and you don't know the person you're addressing - either 'to whom it may concern' or (more old-fashioned) "Dear Sir/Madam". If you do know their name, use "Dear Mr/Mrs/Miss [name]". In informal contexts, "Dear [name]" or simply a greeting such as "Hello!" will do.

I like to start letters in the old traditional way: Dear ABC, (I only write to humans) with the only exclusion of when writing to an enemy or when formally declaring war. It is (for me) a pity that this tradition is fading, and you might even be misunderstood.