Once you click on a lead, you will then have the option to compose an email. In the lead detail view click Email and you can start writing your email.
You can view your previous emails from the leads detail view.
You can also link your existing emails to your existing leads. From your email composer, you can search for a lead to link your email by clicking on Link to existing on the right-hand side of the page.
If you would like your emails to link to deals automatically, you can do this by going to Personal preferences > Email sync.
If you use Pipedrive's Smart Email BCC feature, this also works with Leads. You can read more about Smart Email BCC in this article.