Single left-click the Thunderbird menu button (the three horizontal lines at top right) - a small box will open.
Hover the mouse pointer over "Options" (in Windows) or "Preferences" (in Linux) - another small box will open.
Single left-click "Account Settings" - a bigger "Account Settings" box will open.
Single left-click "Server Settings" in the left hand column - the right-hand side will change.
In the middle of the right-hand side you will see "Check for new messages at startup". There should be a cross in the box at the beginning of that sentence. If not then click that box to put a cross there.
Single left-click the "OK" button in the bottom right-hand corner of the "Account Settings" box.