How to write a trigger to update a field on account – support user whenever a new community user is created in Salesforce?

I am new to Salesforce and want to complete this task.
My question is that i have to update a field on account with name – support uesr whenever a new community user is created. Also the count should increase when the user is created or added and count should decrease when a user is removed or deleted.   Your help will be much appreciated.   Thanks in advance.