Linux MultiMedia Studio(LMMS) is a free alternative to many expensive music creation software such as GarageBand, Magix and FL Studio (FruityLoops). The software is very easy to use, even with no earlier experience of reading or making music, with many free official tutorials for LMMS.
Thankfully, LMMS has support for MIDI keyboards and includes many synth sounds to try out. If you don’t have a keyboard, don’t fear! There is also an option to use your computer keyboard. Download is available from the SourceForge page and is around 19mb and Hydrogen drum packages are also available.