get a hotel and clean up. hit a meeting. i fucking hate to say it, but NA/AA is probably your best bet for a little bit, just haunt the rooms until you can get into a rehab. you can show up loaded, just get it into your head that getting off the shit would be best.
go to a hotel, get cleaned up. shower, shave, shit, and get a good night's sleep. go to a hospital, see if there's a way to get on any sort of insurance plan to pay for rehab.
don't fuck around with just a detox. all that's good for is jacking your tolerance back down so you can go back out and get loaded more efficiently. it works for that, but you're still going to be getting loaded and at the end of that 20k you're going to be amazed at how fast it went and how little you got done with it.
go to a rehab, make sure they have a decent medical detox for your first couple weeks at LEAST, if heroin is your thing. i made the mistake of going to one that only had on-site medical staff for like 6 hours a day on weekdays only. this is decidedly NOT awesome when you're shitting putrid blood because your digestive system has decided to work again after years of being scoured with nothing but vodka and white-box leftovers.
60 to 90 days or longer is best, don't bullshit yourself into thinking a shorter program is better unless that's really how it is.
again, i'm assuming heroin, because that's where i have the most expertise. anything else besides opioids is going to be not fun. if it's H then you've probably kicked before, or at least gotten hopelessly sick for a few days and have an inkling of how terrible it's going to be, even with subutex to get you through it. unless you've got a dime-a-day habit, it's going to suck. balls.
but in the end, you're not going to get shit done with that money unless you get clean first. you will waste it, and you will fuck up any gains you happen to make if you're still strung out, and again, this is speaking from personal observation.
TL;DR: first thing, get clean. creditors, the pigs, the whole fucking world will pretty much ignore you as long as you're in a rehab, and you're not going to get any real shit done with that money unless you've got that out of the way first. i'm rooting for you, bro, i just went through (and am still going through) the same thing minus the check.