If the debt is due to drop off of your report in the next several months because it is almost seven years old, consider waiting until then to pay it, as it will have no affect on your score once it disappear. If the debt shows as written off but will still show on your credit report for longer than a few months, collect all of the funds together to completely pay it off before making contact with the lender. That way, you will potentially re-activate the debt but will also show payment in full, which will minimize the damage to your score.
This story is long winded and all, but the point is, it doesn't matter how bad you have screwed up. It happens to the best of people (I'm an alright kind of guy). But the only way to fix it is to put your foot down, get dirty and fix it. It won’t always be as quick as this and will most likely take a year or more to get in a good place. Then years of maintenance. But if you need a quick hit to your score in a good way, read through your reports carefully (with a credit advisor if you need to. Many personal banks will do this with your for free if you have accounts there in good standing) If it looks like there's something off or something you can fix, call your broker, go over the report with them and STRONGLY insist on a rapid rescore. They will get all your info and see what they can do.
You may also be able to negotiate with creditors as part of a "goodwill adjustment." They may be willing to remove late payments that they've reported to the credit bureaus, especially if you have a history of on-time payments. While you're trying to fix your credit, don't neglect your current obligations. Whatever you do, be sure to pay all your bills on time so you don't accrue any more negative items on your report.
If you do not make your payment on time, most credit cards will immediately hit you with a steep late fee. Once you are 30 days late, you will likely be reported to the credit bureau. Late payments can have a big, negative impact on your score. Once you are 60 days late, you can end up losing your low balance transfer rate and be charged a high penalty interest rate, which is usually close to 30%. Just automate your payments so you never have to worry about these fees.
Your best bet is to call and ask to see if they can put you on a payment plan where you can afford to pay them (even if it’s just the bare minimum a month) or if they will possibly settle for less money. A tip: anything that has your name attached (banking account,utility bills, credit cards, anything you finance, student loans, medical bills, car loans, home loans, your apartment, etc) that you miss a few payments on or don’t pay at all can be reported to the credit agencies and sold to collections companies.