Ice!
If you don't have an ice pack , wrap something frozen out of the freezer (bag of peas) in a tea towel and wrap it around your knee for 15-20 mins at a time. Dont put the ice directly onto your skin or it will burn it.
If you can take ibuprofen, do so, which should help reduce inflammation.
If you have a Tubigrip try that to get compression around the knee.
As CLKD says elevate your leg too.
RICE - Rest, Ice, Compression, Elevation.
Hope it's better soon X