Math isn't so bad when you have someone showing you the steps to a problem. It helps even more when you can rewind and pause those steps to parts that you need to review. Try that with your teacher! Khan Academy helps you do that with thousands of math-related videos that break down simple to very difficult problems into easy to follow steps. The site has thousands of videos so it's easy to get overwhelmed. Just scroll down to the "Arithmetic" section for help on adding decimals or least common multiple. Teens can take advantage of more difficult topics under sections such as Algebra and Geometry.
For the more serious math student delving into deeper math topics in Algebra, Applied Mathematics, Calculus or Geometry Wolfram Mathworld offers a very extensive site. According to the site, Wolfrom Alpha "... works by using its vast store of expert-level knowledge and algorithms to automatically answer questions, do analysis, and generate reports." Plug in a math problem and watch the work get done with a step-by-step solution and a visual representation.