go to global configuration's server tab and then mail settings section.
select mailer as smtp, and other inputs shown below -

• Mailer smtp
  
• From email  This email address is being protected from spambots. You need JavaScript enabled to view it.
• From Name site name
• Sendmail Path /usr/sbin/sendmail
• SMTP Authentication Yes
• SMTP Security SSL
• SMTP Port465
• SMTP Username This email address is being protected from spambots. You need JavaScript enabled to view it.
• SMTP Password  password here of above email address
• SMTP Host  smtp.gmail.com

I used here gmail smtp and entered required inputs, if you want to use your server's smtp
settings to send email using your server's email address, then enter your server's smtp email
address, host, port inputs.

Hope that helped very much, if you have more queries, post in discussions/questions forum here