Here are some affiliate links to products I have used and highly recommend. 

Hook Pad

You can buy Hookpad here

Hook pad is a really cool web applications that lets you experiement with writing songs. It shows you the key you are in and verious other theory behind the music and you drag and drop your music onto the pad. Check it out for free. Pay a small subscription fee to use it. This is a great way to learn a little theory behind writing the music.