You seem to have unvoluntarily disabled Viewport overlays and gizmos (these are all the contour lines, widgets, grids, basically everything that is not renderable geometry). To re-enable them, press these buttons in the top-right corner of your 3D viewport:
The little arrows next to them let you selectively enable/disable elements.
If that's not your issue, maybe it is that you want to enter the Edit Mode?
There are 3 ways to access it:
- On the top-left corner of your screenshot, you can see 'Object Mode'. Click on it and select Edit Mode
- While your cursor is on the 3D Viewport and your object(s) is/are selected, hit Ctrl + tab. A pie menu appears: select Edit Mode
- Still while your cursor is on the 3D Viewport, hit the tab key : it toggles between Edit and Object mode