Shower curtains are a pain! They accumulate mildew and are hard to wash. There are two ways you can help prevent this.
First, you can always pull your shower curtain off and throw it in the washing machine (on a hot cycle).
If you do not like that idea then you can keep a spray bottle of vinegar in your bathroom. Spray the bottom of the curtain (where the mildew accumulates) after each shower. The vinegar smell will dissipate, but if it bothers you then you can add a few drops of an essential or fragrance oil (lavender is a popular one) and that smell will linger after the vinegar smell goes away.
If you do get the fragrance/essential oil then put 3-4 drops on the inside of your toilet paper tube – it will keep your bathroom fresh smelling for as long as you have that tube of toilet paper!