Use "might as well" to talk about doing something that you didn't originally want to, but is now your best choice.
For example, if you wanted to wanted to go to sleep, but you can't sleep, you might think:
I might as well read a book.
Your first choice was sleeping, but reading a book is better than laying in bed without sleeping.
Here's another example:
A: There's so much work to do!
B: You might as well get used to it. That's how things normally are around here.
Read this lesson for a different explanation of how to use "might as well":