How do I track logins, connections, user accounts, queries, ...?
I would suggest you to run a server side trace - for your entire business life cycle. There might be cases where some reports are ran end of month or end of quarter.

So you will need below event in a server side trace 11,13,16,19,33,37,61,162 with these columns 1,6,8,10,11,12,14,27,35,40.

You can filter out stuff that you dont need using sp_trace_setevent (note that it is announced deprecated).

Alternatively you can use Extended events - Tracking SQL Server Database Usage.

Note: Currently in my environment, I am using Server side trace and its a very low impact. Also, I have a job that dumps the profiler info into a table for analysis.

You can have below table structure for loading trace data :

CREATE TABLE [dbo].[login_trace](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [ServerName] [varchar](300) NULL,
    [HostName] [varchar](300) NULL,
    [ApplicationName] [varchar](300) NULL,
    [DatabaseName] [varchar](200) NULL,
    [LoginName] [varchar](100) NULL,
    [StartTime] [datetime] NULL
) ON [PRIMARY]


You can adjust it as per your requirement.