First locate where java is installed, and where javac.exe is located.
This will probably be something along the lines of C:\jdk\bin\ or C:\Program Files\Java\jdk\bin\
Second, go to Windows Explorer, right click My Computer, go to Properties, the Advanced tab, Environmental Variables.
Under System variables, look for Path. Do NOT remove anything here, but at the END add a semicolon (;) and put path till C:\Program Files\Java\jdk\bin\
Remember put correct folder name.