Learning to code, coming from an experimental background, can be a frustrating and intimidating experience. Although there are many free courses online that will teach you the basics of Python and programming, the best way to improve is to simply practice. The best practice is a project you are personally motivated and passionate about. For graduate students, this might be a component of your thesis or a side project that complements your research interests. Coming up with an interesting project might be daunting at first, however, so here are some resources for quick, achievable practice problems to help keep you coding.
Practice mathematical/computer programming problems
Python basics with Rosalind.info
Beginner bioinformatics with Rosalind.info
Check your code style online