How to fill the custom fields of users from the front-end form? how can i fill custom fields from the front-end form ?

https://craftcms.com/docs/templating/user-registration-form
You can do so by using the fields[customFieldHandle] syntax.

For example, here's how you'd populate location and bio custom fields.

&lt;label for="location"&gt;Location&lt;/label&gt;
&lt;input type="text" id="location" name="fields[location]" value="{{ currentUser.location }}"&gt;

&lt;label for="bio"&gt;Bio&lt;/label&gt;
&lt;textarea id="bio" name="fields[bio]"&gt;{{ currentUser.bio }}&lt;/textarea&gt;