The question is a bit broad and this answer assumes that you want to write an email using a computer (as opposed to using other forms of technology which may also provide email-like functions).  You need to use software designed for this purpose, commonly called an 'email client'.  This software can reside on your computer (eg. Microsoft Outlook) or it can reside on the internet (eg. 'webmail').  You would follow the instructions within the software application to create a 'new message' (typically by clicking on a button) to open a new (blank) email template.

The following process, outlining the minimum basic requirements is recommended:

1) Type your message, including sign-off, eg. your name (like a letter, email is formal written communication, so use an appropriate writing style)

2) Type in the email address of the 'recipient' in the appropriate field (usually in the section above the message area in the email template).

3) Type a short phrase to describe the 'subject' of your email in the relevent field.

4) Proof-read your message, recipient details and subject and make any necessary corrections.

5) Click 'send'.

Mission accomplished (assuming you have internet access!  Otherwise, it was all a waste of time really!)