First of all check if you don't have an M2_HOME environment variable and then just follow the steps:
1) Start by downloading maven 3. Install it in a known directory like c:\tools
2) Next, set your PATH to point to the bin directory. If Maven is in c:\tools\maven-3.0.3 then add c:\tools\maven-3.0.3\bin to PATH:
3) Maven should work now; if not, then you've skipped something or it’s a problem with your configuration.