You need Three Tools to decompile an APK
file.
Dex2jar - Tools to work with android .dex and java .class files
ApkTool - A tool for reverse engineering Android apk files
JD-GUI - Java Decompiler is a tools to decompile and analyze Java 5 “byte code” and the later versions.
for more how-to-use-dextojar. Hope this will help You and all! :)