If you publish research done with oTree, you are required to cite this paper. (Citation: Chen, D.L., Schonger, M., Wickens, C., 2016. oTree - An open-source platform for laboratory, online and field experiments. Journal of Behavioral and Experimental Finance, vol 9: 88-97)
If you will use oTree Studio (easiest option), go to otreehub.com.
Read: why you should use oTree Studio.
If you are an advanced programmer, you can use oTree with a text editor.