As mentioned earlier, immersion is key. You can make Spanish a priority by reading/watching everything in Spanish, speaking with native speakers, and experiencing the culture nearby (if there is any). Mastering a language can take a while, but if you get the opportunity to travel and/or live in a Spanish-speaking country, you can be fluent pretty quickly! Learning colloquialisms, picking up the local dialect, and blending with the native population will do that pretty nicely. If you are currently attending college, you can take Spanish classes if you have the time.
Your community may have resources to help you learn Spanish as well. Where I live, there are a lot of resources for learning Spanish (as I live pretty close to Mexico), and Spanish is pretty much the de facto second language for many people (and for businesses as well), so picking up the language is pretty easy.
I took French in high school and, like you, want to become fluent. I'm kind of slacking off on the immersion part, but I'm trying to get my speaking skills up to par if I ever want to go to France for future study abroad programs! Right now, I listen to French radio in the car and have gotten the hang of translating French articles into English. Some things I probably won't learn entirely, like complicated slang and such, but I hope to make something out of learning a second language.
Good luck and have fun learning Spanish!