There are two ways that users* can send an email to a single contact:

Send via Quick Send

Start by navigating to the Emails menu. Create a new email, or click on an email from your saved list.
From here:

• Once in the email editor, click Quick Send (if you have just created the email you will need to click Save first to enable the Quick Send button).

• Within the Choose recipients module, the Search for Contacts, Tags & Audiences option is the default.
  Start typing in the name or email of the contact you would like to send to (you may need to type/paste in the full email address to find the matching contact record).
• Select the correct contact from the list of matches in the dropdown.
• Add more individual contacts if you wish by repeating this process.
• In the Date & Time module, select when you would like to send, e.g: Send Immediately.
• Click Send

Send via Campaigns

Start by navigating to the Campaigns menu. Create a new campaign.
Once in the campaign builder:

• Within the Choose recipients module, the Search for Contacts, Tags & Audiences option is the default.
  Start typing in the name or email of the contact you would like to send to.
• Select the correct contact from the list of matches in the dropdown.
• Add more individual contacts if you wish by repeating this process.
• In the Date & Time module, select when you would like to send, e.g: Send Immediately.
• Click Send

Send to more than one type of contact

If you would like to send to more than one type of contact - e.g: an individual contact and a tag, just select both:

• Send to contacts that have a certain tag
• Send to an audience or audiences

*Admins have the options to Send to All Contacts, or Audiences