To start with, look up the contact under Contacts > Contact lists. To search between all contacts, click on All contacts, then use the search box.

To view the contact in edit mode, click on the name or email, and a “contact card” will open showing all the details. You can edit contact details, add or remove lists and links. In the Activity section you can view the history, including status for all emails that so far have been sent to the contact. 

Please also see:
How do I add contacts?
How do I add or remove an existing contact to/from a contact list?
Where do I add or edit a contact on my public newsroom?