This a model of email and some tips that will help you with this writing part:
- Say how are you (in case your friend asks you) and ask him/her the same (" I'm fine, thank you. And how are you?"
- Always reply the questions he/she asks.
- Try to add another sentence to that reply that complete
- Be careful with the tense
- Close you email with formulas as "see you soon", "write me back"...