@Yann de Champlain

In CRM there is no Tasks, there are Leads and Opportunities or Issues if you install the corresponding modules.

Every opportunity in the system has an associated 'Next Activity' - such as an Email, Quote, Call or Task.

These can be defined by Sales Team, can be chained together, and can have default numbers of days between them:

You can create any type of Activity, and series of Activities that match your process for closing Opportunities. 

Activities show up on the dashboard - so you are always reminded when you said you wanted to complete them.

(Although the screenshot is from the Enterprise Edition, the same functionality is available with the Community Edition).

If you still want Tasks, install the Project module.  You can create an Internal Project and add Tasks to that.