I'm new to joomla custom fields and also don't have a deep knowledge of joomla. I need to create a custom fields name "Speakers" to associate to a event. It's a multiselect field so that when inserting a article there should be the possibility to add multiple speakers. But, besides that, I need the possibility to manage the speakers, so that if one new speaker appears it could be added. Certainly that this is very easy to do, but as I said I don't know much of custom fields. Thanks for any help.
You can extend the Tag field provided in native joomla and build your own field on top of it.
A sample tutorial on how to build a custom field is provided here https://docs.joomla.org/Creating_a_custom_form_field_type .
Assuming that you are building a custom component for your events management, also take time to go through the code in these files:
You will get ample knowledge on how to go about your problem.
A sample of the tags field in usage is found when editing an article. Go to content manager -> Articles -> New Article and look for the tags field in the right.
If that's too much work and you don't want your users to add new speakers to your database, then you can simply provide a list of predefined speakers and let users select from that through a normal select field.