Writing a formal email

Look at these links that will help you handle the language you need to write a (semi)formal email.

This one is a very thorough but straightforward explanation of formal emails by the British Council. There are some tasks you can do at the end of each section.

Here's some advice about different language for formal/informal emails.

And here there's some more advice and an exercise at the end.

And of course, whenever you write anything in any language, bear in mind this and this.