Sitecore Forms module on Sitecore 9 Initial release comes with only 5 Submit Actions OOTB.
Trigger Goal, Trigger Campaign Activity, Trigger Outcome, Redirect to Page and Save Data
To send email on Submit action either you can use Marketing Automation and send emails by selecting “Trigger Goal” submit action.
Else you can write Custom Save Action to Send Emails.
Else Sitecore 9.0 Update 1 comes with one additional Submit Action – Send Email Campaign Message, try exploring that.
P.S. I haven’t tested the new Submit Action yet, assuming it should work without any issues.
Update 1:
Also have a look at this Custom Send Email Action, also explaining the issues with OOTB Send Email Campaign Message action with Sitecore Forms:
http://www.nishtechinc.com/Blog/2018/April/Send-E-mail-Action-to-Sitecore-9-Forms