Check out these examples below of creating songs from an idea:
https://www.youtube.com/watch?v=OEIk3bJtTNw
https://www.youtube.com/watch?v=u0K_xcSqhxQ
https://www.youtube.com/watch?v=B7gLskvda3s
https://www.youtube.com/watch?v=LWQVztiJHfs
https://www.youtube.com/watch?v=ZOKB44woiU8
The composition handbook takes you from the beginning with having an idea, to the very end where you start to add advanced features to do with dynamics, articulation and ornamentation.
Creating an idea is just the start. Once you have an idea then you need to, at the very least:
Decide on a tempo (speed)
Think about what instruments you wish to add, if you want more than one instrument. You can just compose for one instrument though...
Think about a structure - is your idea the introduction? The verse? The ending?
Think about other parts such as bass, or chords, or melody, or rhythm. Your composition booklet (linked above in the title) explains the three key components of almost all music
Rhythm
Accompaniment
Melody
There's much more to think about but these are some of the questions that most composers have to think about at the very early stages. It's very important that you READ the composition handbook very carefully to help guide you.
Loopy HD is an app for iPhones and iPads allowing you to record multiple loops at once. There's much more to it than just recording loops. Click here to see it in action.
The GarageBand app (not the desktop version for Apple Mac computers) is a very powerful and comprehensive personal studio. In theory you can record an entire song/piece using this app alone. It is available for iPhones and iPads only and is free. Here is a demonstration of GarageBand (iOS version)
Bandlab is very much like GarageBand but is not just limited to Apple devices. Bandlab is available as an internet program meaning you simply require a device (Mac, PC, tablet, phone) with access to the internet. Just like GarageBand, Bandlab allows you to record an entire piece with everything from drums to strings including your vocals and live instruments. It is also available as an app and is free.
Just like BandLab, Soundtrap is an online internet based program with the ability to add vocals, instruments and play parts into your arrangement.
Mainly for electronic music producers, Audiotool is just like BandLab and Soundtrap expect that you drag your instruments into a 'space' and control them directly, rather than entering notes or MIDI notes into a timeline. You can have tens of instruments all at once. Audiotool is 'loop based' software. You compose a 4, 8 or 16 beat loop and you then manipulate your loop by adding or subtracting parts and adding FX.
Musescore is a free notation application. You can use notation software to compose your piece and have the software play it back for you. Click the link above to download Musescore for FREE
Soundation is just like BandLab and Soundtrap, but again mainly for electronic music production. The features are much more limited though with 'free' option. There are lots more features with the paid options but for just creating ideas it might not be worth paying for those features.
Here's an explanation of rhythm that you might find useful before checking out the one linked below: https://www.youtube.com/watch?v=2UphAzryVpY