Khan Academy
    Kahn Academy is a great online resource for many math topics.  You can find video lessons and countless exercises on almost every topic you will cover in high school.

    Wolfram Alpha is an amazing resource that will solve any math question you give it.  It requires a paid subscription to show the step by step solutions to truly complex exercises, but it is generally an excellent resource to double check your solutions.

    Desmos is a free graphing calculator run through Google.  You can quickly solve equations graphically and do lots of other neat things.

    There are a lot more than cat videos on YouTube.  You never would have guessed it several years ago, but now there are teachers all over the world carefully crafting educational videos on a wide range of topics, just search!