when any one registeres in my site it doesn't send mail to user that he is registered successfully.
You need to install below Gmail Smtp module to magneto 2 store for working email properly in magento local instance.
You need to follow all the steps from above module README.md files and setup as per guidelines.
After installing extension and setup from configuration your local instance mail is working fine.
First up do any emails get sent not just new user registration?
If NO then it looks like your server is not setup to send email. You have 2 options here. First up is to install sendmail, depending on the hosting env it could be as simple as `apt-get install sendmail` and ensuring it starts. If its managed the then provider can install and configure.
Alternatively you can use a service MailChimp, MageMail etc that can relay emails on your behalf or use the SMTP plugin that also relays your emails to a dedicated email server.
On my installs I prefer the first option as its the most straight forward for most applications.
If you need more help please let me know. More than happy to assist.