It's best to find some software who is suitable and easy to use for you. I use Fruity Loops from beginning of my production journey. Some programs who are mostly used by popular artist are:
- Cubase
- Ableton Live
- Reason
- Fruity Loops
Also I saw many artist using Ableton Live on MAC and it's most popular program now. But it could be little tricky to use until you spend some time.
Youtube is full of tutorials about all of them.
I can give you a strong recommendation to try Reaper. It's free to try fully featured and has strong midi implementation. You can also try different skins which emulate different DAWs. Ableton is unquestionably the best for composing on the fly ... it lets you run a standard scrolling DAW in the background, capturing what you are a triggering... while you launch "clips" of midi or audio, all time sync'd. Basically a perfect hybrid of DJ sampling workflow, midi notating and DAW editing.
But yeah. Reaper is free. And also great. So there's that.
Thanks for recommendation. Seems that it's very lightweight as it's only 11mb for Win-64bit.
But also looks very interesting.