Know your best and favourite learning medium. Then use it and be in it.
My favourite way to learn a language, and as a result, the culture itself, is to be immersed in the country where the language originates.
Mandarin is primarily spoken in mainland China as well as Taiwan (R.O.C.). You will also notice at your city's Chinatown (if you have one) that the owners and workers there will speak either Mandarin or Cantonese, usually.
What is your favourite learning medium?
Are you a classroom-textbook-contextual-teacher-student learner? Perhaps signing up for Mandarin classes at your local community college is a good option.
Are you a one-on-one private learner? Perhaps finding a tutor to speak Mandarin with is a viable option.
Are you a drop-me-on-the-Great-Wall-from-a-helicopter-and-see-if-I-survive type of person? Aka deep and immediate immersion? Then perhaps living in China (or Taiwan) is a preferred option.
You know yourself best. You also know how you learn new languages the best.
And even if you don't, try out all the various learning methods out there. See what works. It's never too late to start learning a new language in whatever medium you prefer.