How to use the Cruise Control on Honda Civic?

After i press the cruise control button and took the leg off of the gas pedal the car is slowing down.

Seems like you've just got the Civic. It's manual transmission right? Is there an Owner's manual in the glove box? Since you've not specified the year for the Civic, I can't tell you what to do or check the cruise control system. If the fuse has power and good:

It's the easiest cruise system I've ever used and in my car it works perfectly.

For those new to it or still waiting for the car:-

Press the inner most button (cruise main) to switch the system on.

Once at the desired speed press the down button part of the middle rocker button (Cruise set).

If you touch the brake or clutch at any time the Cruise will deactivate instantly. To resume at you last speed press the up part of the middle rocker switch (Cruise Res). The car will now gently accelerate up to the set speed again.

The right hand button is cancel. This obviously cancels the current cruise selection (same as pressing the brake or clutch). It does not turn the system off. To do that you need to press the Cruise main button again

If you wish to increase or lower this speed while the system is set at a speed you can press either up or down on the Rocker button. You can accelerate whilst in cruise to overtake but once done the car will slow down gradually to the set speed again, so the throttle does not increase the set speed (obvious i know)

The system will not work if you are even slightly on the clutch pedal or brake (I know people who rest their foot on the clutch while they drive thinking its not sensing it, if the Civic can sense even the slightest touch and if it does it won't active the cruise).

Helps if you press SET. Now, if that doesn't work, a vacuum line is broken or disconnected.

You should learn to READ YOUR OWNER'S MANUAL to find out how to correctly and safely operate your vehicle, Einstein.