Adding the domain account to a security group on the SQL Server computer that has sufficient privileges to log on as a service
To answer your questions directly:

2) In order to add a login to the sysadmin fixed server role you can utilize ALTER SERVER ROLE ... ADD MEMBER (2012) or sp_addsrvrolemember (pre-2012).  Here would be an example:

exec sp_addsrvrolemember 'yourLoginName', 'sysadmin';


3) Without having to reiterate what's found in the following documentation, take a look at this MSDN reference on the service permissions for each SQL Server service.