How to open the radar in CS2

The radar in CS2 is on by default. It lives in the top-left corner of the HUD and refreshes continuously while you play. If you can't see it, it's almost always a HUD setting or a display mode issue - not something you have to enable.

The default - it's already on

Launch CS2, join a server, and the radar is in the top-left. No setting to flip, no command to type. The dot in the middle is you; the coloured dots are teammates; recently-spotted enemies appear briefly when a teammate sees them.

If the radar is missing

Three things to check, in order:

HUD visibility. Press ~ to open the console and type cl_drawhud 1. If you (or a config you imported) had set cl_drawhud 0, the whole HUD was hidden, radar included.
Radar size. The HUD radar can be scaled small enough to feel "gone". Run cl_hud_radar_scale 1.15 to push it back to a normal size. Anything below 0.8 is hard to read.
Display mode. On some setups, switching between fullscreen and windowed mode can leave the HUD render-state stale until a map reload. Disconnect and reconnect to a server.

How to enable the developer console (if commands don't work)

Settings → Game → Enable Developer Console → Yes. The default key is ~ (sometimes labelled ` below Escape). Open the console, paste a command, press Enter.

How to make the radar more useful

Once it's open, the single highest-leverage change is turning off "always centered" so the whole map fits in view:

cl_radar_always_centered 0
cl_radar_scale 0.4

On smaller maps (Mirage, Inferno) this fits the entire layout on the radar at once. Full reference: CS2 radar settings and commands.

What the built-in radar shows you

Your position (the centered dot, or wherever you are on the map if centering is off).
Teammate positions.
Enemies recently spotted by a teammate - they fade out a few seconds after the spot.
The bomb dropping / pickup / planted state.

The radar does not show enemies you haven't seen recently. That's by design - it's part of the game's information rules. If you want a top-down view that shows every player continuously, that's a different category of tool. See what is a CS2 radar?