LMMS stands for Linux Multimedia Studio, but despite the name, this is actually a powerful Windows tool that lets you compose, edit and mix music with the help of your computer.
LMMS includes all the tools you need to create music from scratch (both melodies and background beats), mix and arrange samples and prefabricated sounds and finally producing and exporting the results. What's more, you can connect your MIDI keyboard to the computer and play with it and LMMS – or simply use the onscreen piano roll.
The program features a sleek dark interface that you will likely find overwhelming the first time you run the program. There are loads of menus, tools and other information to pay attention to. Fortunately LMMS is also extensively documented, including not only a complete wiki page about it but also dozens of tutorials and ready-made tracks to get you started.
LMMS features also a bunch of samples, drum loops and other creative material you can use on your compositions. The program is fully MIDI-compatible and its functionality can be easily expanded with plug-ins.
If you like composing, mixing, editing and playing with music on your computer, this is the application you're looking for.