The #1 thing you can do in the next 12 months is pick the right schools and write killer essays. The "what can I do in the next X months" question usually gets answered that way because the truth is that the prior 28 years of your life is going to vastly outweigh anything you do right before you apply. Your best chance of getting into a top 20 school is to:
A) pick schools that are favorable towards career changes and that have the structure to support someone with finance goals. This is, fortunately, a big list. I would avoid schools with extremely high average GPAs and extremely low acceptance rates (Harvard, Stanford, Wharton, MIT, Haas), I would avoid Columbia (not favorable at all towards career changers), and then I would pick from the rest. Booth, NYU Stern, Kellogg, Tuck, Ross, Duke, Cornell, Darden, and UCLA all come to mind. With your GPA you will want to apply to at least five schools to mitigate for the risk of some of them frowning at the low number, but that's a nice healthy list to pick from.
B) showcase your maturity, focus, and discipline in the years since college (through the essays; this usually requires the assistance of a vetted consultant, to be blunt)
C) write perfect career goals essays that showcase a clear understanding of the power/limitations of an MBA, a clear path from where you are today to an MBA program to your short-term goals to your long-term goals, passion behind what you want to do long-term, and a very clear explanation for the transferrable skills you have from your sales work and how they will appeal to recruiters in the finance space. Everyone always wants to know if doing X will hurt then in trying to do Y, but the truth is I don't know the answer until I see a goals essay. If you tell the reader (and, by proxy, the recruiter) why you will thrive in job Y, thanks to job X, you are fine. If you don't walk the reader through that, then you are screwed. It's that simple.
Now, aside from the essays, what else can you be doing?
For starters, you should take a finance course or two. Perhaps look into the MBA Math series, or even just take some community college or online extension courses. If you want to be taken seriously as someone interested in and capable of doing well in finance, you need to have some evidence. If your grades were poor-to-mediocre and your work doesn't show strong quant/finance skills, you have to find a way to showcase them.
I wouldn't try to join a bunch of clubs or anything like that, because it will be completely dismissed as a desperate attempt to pad your resume. Schools barely care about laundry lists of activities anyway, because they are hard to vet and hard to normalize (across industries, age ranges, regions, etc.).
As for waiting to get experience, it all depends on what kind of experience and what kind of transferable skills you can develop (see above). Getting a few years of experience just for experience sake may not help at all. If the job is not a good one, it will hurt you more than help. If it's a great one, they might wonder why you need an MBA if you were able to switch careers into the job you wanted on your own. I would probably suggest going all-in with what you have now (plus the finance courses) and just do a fantastic job of telling the reader what it all means, how it all makes sense, and why they should admit you, basically.
Hope that helps.