Before you go any further here, I just want to remind you that it's pretty unlikely that someone has cast a negative spell on you unless you have other witches or Wiccans in your social circle. This isn't the kind of thing that non-Pagans just whip up to get on your nerves. Do a little research into the problem before you jump to conclusions.
That said, techniques to break a spell are intended to do just that. Break a spell. That means they will do nothing if there is no spell on you in the first place, so you don't have to worry about inadvertently harming someone if you were wrong.
A Clean Break Spell
A good symbolic break can do wonders to snap a negative spell against you. You should have:
Any type of wood will do, but the branch need to be dry enough that it will break when you bend it. Wrap black yarn a few times around each end of the stick, and as you wind the yarn, wrap in your herbs. Then attach a safety pin to the yarn at each end.
When the stick is done, leave it on your altar overnight. The next day, pick up the stick, holding the yarn-covered ends in either hand. Say loudly, "I break your spell, you are broken!" and then snap the stick in half. Carry the two pieces of stick around with you for 3 days to remove any remaining traces of the spell.
You can see how it comes together on our Youtube channel:
Broken Glass Charm Bag
This little charm bag is carried around with you to help protect you from the effects of the negative spell. After 7 days, the spell should be broken completely. Get your stuff together:
Twist up the bit of wire into a knot and drop it into the bag, saying "You are broken and have no power over me". Repeat the same words as you drop each piece of glass and stone into the bag. Tie the bag shut, also repeating the same words.
Now you need to have this bag with you for 7 days, including near your bed when you are sleeping.