Hi Rebecca, look for a bush killer containing either Glyphosate or Triclopyr. You should find them at your garden center. Brush it onto a freshly cut stump of the tree and they'll die.
For many rock salt is a killer. For large trees remove the bark in a ring around the trunk. Takes time but it will kill it. If it's gotten too big a chain saw is the solution. You may have to take it down in sections so it does not damage anything by falling on it. Be especially careful about cutting around electric wires.
I boil vinegar and pour it on what i want to die. Just be careful, it'll kill any grass or plants it touches