Hello guys,
I badly need advice on how to do this for our science project. Our project is to monitor garbage through radio waves and sensors... the problem is that how do we connect the sensor to the transmitter and how do we get to program it to transmit the garbage levels... and one more thing... What Type of sensor do we use?

You really should start your own post with this as other then communications it's a different project!

You first need to determine how you are going to detect trash level. Keep in mind a particle of trash isn't always small. And large skinny objects quite often get put into trash cans. That hindged or removable lids can fly off. That by nature trash cans are filthy so sensors will be easily disabled! Top of my mind ultrasonics but I'm sure someone else may have an alternate sensor. Anything with light will get contaminated! Wide angle LED with three or four sensors across the trash can. One of these about every foot or so? A skinny obstruction will only block a single sensor and not all. A fuller can will block out all sensors each level! Also a tilt sensor to detect the can is mostly inverted so when it is uprighted again, the sensors get recalibrated due to being obscured by liquid/gooey waste materials!

A thought is a couple high intensity LED's near the top of the can tipped downwards bathing the entire can in light. Sensors in the side walls at various levels detect that light. A trinary pair at each level might suffice!

If you don't know electronics, then you'll need to try to find an off the shelf sensor that will plug into your computer! Or into an embedded board that communicates with your computer.