Best thing you can do is settle those debts. When they have gone to a collection agency you should be able to settle on a lesser amount than what was originally owed, sometimes this can be knocked down by 75% of what was originally owed, if they are unsecured debts. Secured would mean it is secured by a house or vehicle. If they are unsecured it would be a credit card.
Be sure to gather paperwork that says the debt you are trying to settle will actually be settled, ask for paperwork ahead of time and then once you agree send them money to pay it off, preferably a cheque or money order to show that it has been paid off. This now gives you 2 pieces of proof you have paid it off.

As for strengthening credit, probably the best thing to do is simply manage your credit cards better. Don’t buy anything you can’t afford to buy. When you charge something on a credit card, put the money away until the statement comes and then pay it off.
Worst things to do, is to miss payments. Do not miss payments.
Keep balances low and try to pay them off to zero monthly. If you carry balances it also counts against your credit score.

Basically, follow these rules:
1) Don’t buy what you can’t afford
2) If you buy something on a credit card, set the money aside so that you have the money to pay off the amount on your credit card statement when it comes at the end of the month.
3) Pay the full balance off monthly so that you are not carrying a balance. This way your credit score is maintained and you don’t pay any interest charges for carrying it on your credit card.

Victor