There is more than one way to express it. I believe 'con fluidez' is the most usual way to express this. Native Spanish speakers jump in if I'm wrong. 'Quiero hablar español con fluidez.' 'Hablo inglés con fluidez' etc. I want to speak Spanish fluently. I speak fluent English. 'Quiero hablar bien el español' would be another option. I want to speak Spanish well. If you're talking about wanting to master all facets of the language, and not just speaking I think 'Quiero dominar el español.' is a good option (I want to master Spanish.)

Paul's answer couldn't be more correct! And you can trust me since I live in Spain (maybe you already noticed because of my writting...)

Paul said everything that needed to be said. I can only think another possibility: "de forma fluída" that would mean "in a fluent way". I'm from Spain too, native speaker so :). Great Paul!!

As a native speaker, I can't agree more. 'Hablo inglés con fluidez' is correct. However, "hablar muy bien" or "hablar como nativo el español" are some more common expressions.

Quisiera hablar con mayor fluidez.