Collect any of the remnants of the spell: anything written, any candles, ashes, herbs, whatever - then take them to a crossroads and leave them there without looking back - or dispose of them in running water. Return home a different route, going directly home without taking to anyone, praying the whole way for a reversal of the situation, whatever it is.
Go home and take a spiritual cleansing bath. Google it for techniques and look for easy recipes - salt water + holy water works. Finish your bath and light a blessing candle for you and for whoever else you have potentially affected/damaged, in whatever way. Continue this for seven days in a row.
Once you start this process, stop obsessing about whatever it is you did. Stop thinking of anything but positive things about you and whoever else is involved. Don't feed the situation anything but Light.
You'll be fine. Magic is but energy, and it fizzles out after time - usually much quicker than people fret and brag about.