You can use {{lead.First Name}}
In the email designer, look for a little blackboard, that is the token lookup dialog. You can access most fields from that as a token.
But remember that if they type in "Ben" as their first name, and the database previously had "Benjamin", their new data will overwrite the old data, *unless* you field block First Name.
If you field blocked First Name, then the token would render as the original data, "Benjamin"