In Sql server you can login in two mode:
1. Windows Authentication
2. Sql Server Authentication
But you can only access SQL Server in both way by default if you only have selected both the login option when you install the SQL server.
But, If you forgot to select both the option while installing the SQL server, you can edit the option in following way:
1. Right Click on Server instances and view the Properties
2. Form the Property window and list at the left select the “Security” tab:
3. Select the option indicating both the login mode and Click OK.
4. Cheers!! 🙂