Your average credit card carries a 17% interest rate, so it is not a good idea to max it out. However, if you have already maxed it out, another option is online loans with bad credit. There's plenty of enterprises that will offer you a loan if your credit score is not exactly the best, with the quantity and the interest rate varying according to the institution that provides it, the type of loan you want, the loan term and the amount you decide to pay monthly. Quick research on Google will tell you which institutions offer this option, but research carefully!
You have plenty of ways to make a quick buck to be able to survive, but this won't be able to keep you afloat for long if you don't learn long term strategies to make out the most of what you have. If you keep struggling financially, then it would be wise to spend some time learning ways of making the most out of the little money you have; and this is possible for anyone, whether they have earnings, savings, or none. Read books about finance, take classes, learn how to stay frugal and learn how to manage your debt and keep your credit score up.
If you have both earnings and expenses, you should also have a budget. A budget will ensure you always have your spending under control and that you manage it correctly, and if you have enough earnings, it should even grant you a way to start saving.
Sell Plasma
It's legal and possible in most of the world. The requirements are easy to meet (be of age, healthy, don't do drugs, drink or smoke, be over a certain weight). You can expect anything between 25 pounds to 50 pounds per donation depending on how much you are able to give, and your health will determine how many times a month you are able to donate. You can easily find on Google a list of plasma clinics near you and get screened for eligibility.
"Beermoney"
Not enough to make a living, but enough to pay for beers, hence, "beermoney". This is the name given to different easy, paid tasks online like filling out surveys, watching videos, solving captchas... Most of them have barely any requirements other than a computer or smartphone, an Internet connection, an joining a site. If you want to start, just search "beermoney" on Google.
Garage Sales
Nowadays, you can do them physically or online. If you'd rather do it online, just list your stuff on Craiglist or even apps like Wallapop that allow you to sell your stuff without a listing fee. If you can do it physically, all that's needed is some room and some advertising.