were the debts from overspending/disorganization? or was there something like medical bills that were out of your control?

you might want to check out Dave Ramsey's plan for basic budget boot camp and guidance.

>can you put any amount in?

depends on the company. it's not unusual that companies require a minimum $1000 or $2500 for their funds, and each purchase thereafter needs to be $100 minimum. Fidelity has the lowest minimums for mutual funds IIRC. $0 to start and $10 is the minimum bank transfer.

ETFs are different, as mentioned.