How can I say ((nice to meet you)) in German?

Schön Dich kennen zu lernen. (if you don't know the other)
Schön Dich zu sehen. (if you know the other)

Hello Ahmed,

When you meet someone for the first time by making an acquaintance:
"Ich freue mich , dich (Sie) kennen zu lernen."

Otherwise when meeting someone you already know ,a friend or so:
"Schön dich zu treffen."

In a formal way you could also say when meeting someone :
" Guten Tag ( Abend) Sehr angenehm!" = pleased to meet you!

If you writing a formal letter for example you could be more formal saying :
" Sehr erfreut, ihre Bekanntschaft zu machen."

Freut mich!

"Freut mich" is one possibility, beside the more formal answers which Cherry gave.

A bit oldfashioned, but formally correct is short "Sehr erfreut!"