On each tab - Tasks, Leads, Delegation, Opportunities, Accounts, Contacts, Appointments, Call List, Insights, My settings - there are sub-tab screens, you can set what sub-tab screen you want to display as start screen by following steps:

1. Go to My settings

2. Go to Personal tab, you will see Display Options section, where you can choose what you want to see as the first view

Go to each tab to check the result.