Writing letter to penpal

Hello,

I would like to ask about opening and closing sentences for informal letter written to pen pal.

It does not have to be traditional style. I know they are very difficult.

I can only find formal letter templates/suggestions.

If you can write an example of letter opening and letter ending sentences I would be very grateful.

Also, are there any major differences based whether sender/receiver is girl/boy man/woman.

Or anything else that I should know when writing letter in japanese ...

In what language are you writing to your penpal?

As she said in the last line.

Casual letters (or e-mails)these days do not require any specific form.
You can write just you do in English.

Begin with "xxx (your penpal's name)", which is followed by "konnichiwa" and/or "ogenki desuka?".
At the end of the message, you can write "dewa matane".

Please note that there's no real equivalent to "take care" so "matane" (meaning next time) will do.

Hope this helps.