I need to do a link witch can start a process model. How I can do this?

You can publish the Process Model as an Action and the link will be available in the Actions tab. Do you mean other than under the Actions tab, or other than a Related Action on a Record Dashboard?

I need to start a process from a link in an email

I mean I have an approvement process witch end in an email sended with a link that once pressed a person can make finish the process. Is it possible?

I believe so. You will most likely need to publish the Process Model as a webservice, which will give you a link to the process model which will start the process. You can put that link in the email. I believe the inputs to the process are placed in the url which limits what you may input to the process model using this method. Hope this helps!

You cannot start a webservice from a link in an email.

But you can have the person reply to this email and have it sent back to the Appian system. Either starting a new process or add a reiceive message event to the model and have it wait for the email.

I tried your use case and its possible to provide the action URL on the Email message body.
For Example : The URL for "Apply Leave" Process is dev.appiancloud.com/.../abc1234, then you can provide the same in the configuration of the message body of the "Send Email" smart service, and the user will be prompted to the Action's form.
We need to make sure that the URL should be updated with respect to different environments.
To do things we can make it possible but its better to consider the Appian best practices, not sure if this one's allowed.

For this to work the user receiving the email would have to have a valid Appian user account. But then you could just assign a task to him. By default he would receive an email including a task link he could click on.
As I understand the use case this is about a person without an account who will not be able to directly take any action in Appian. As I said, integration via email reply is easy.