Open a terminal and type:
tar xvjf filename.tar.bz2
Which will untar it to the current directory. Normally (99% of the time) it will create it’s own subdirectory so you don’t need to worry about that.
Just so you know:
tar – Tape ARchiver
And the options:
x – extract
v – verbose output (lists all files as they are extracted)
j – deal with bzipped file
f – read from a file, rather than a tape device
“tar –help” will give you more options and info