There are two ways you can send an email to all* your contacts:

Send via Quick Send (users)

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, tick Send to All Contacts
• In the Date & Time module, select when you would like to send, e.g: Send Immediately
• Click Send

Send via Campaigns (users and admins)

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

• Admins: Choose the users you want to send on behalf of.
• Within the Choose recipients module, tick Send to All Contacts
• In the Date & Time module, select when you would like to send, e.g: Send Immediately
• Click Send

* It's important to note that whilst you have selected "Send to All Contacts", your actual total number of contacts reachable will exclude those that are unsubscribed, deferred, and any contacts on your blacklists.