Well, I can't tell you from knowledge, but I can tell you from what me and my "business partner" are doing right now. Basically, that is whatever you can.
Someone might want to back me up on this, because I only know from internet research, and this is possibly not the most reliable information. However, you might want to keep these things in mind.
First, no financial institution that I know of (i.e. banks) can or will give you a loan if you are under the age of 18, just because you can't be legally held into a contract. You suggested that you were over 18 though, but I wouldn't keep that idea in mind either. You could ideally get a student loan if you are in college (possibly if not), but not directly for college (my friend paid off a car with his, and he was still able to defer payments for 6 months).
Secondly, there is good news and bad news. The good news is that, all facts considered, most small business loan requests are for roughly $175k - $750k, and anything under $30,000 is considered a very small loan from the banks view. The bad news is, very small loan or not, if you are 18 or 19 (or whatever, you get the idea) years old, you probably have just an OK credit score, and nothing of major value in your name that you could put up for collateral. This means that banks are very unlikely to give you this loan, even with an absolutely spectacular business plan, just because you have no history, and nothing that they can take on the chance that your business goes under. They don't want to be stuck with a multi-thousand dollar bill that you can't pay.
You might be able to find some funding through loans if you find a willing co-signer, but then, they need to have good credit and something as collateral (if your parents co-signed a $5,000 loan, you may not get the best interest rate, but they generally have cars, equity on the house, etc.) in your place.
Also, you might want to look into some business grants. However, if you find some that work out for you, let me know, because I've been unsuccessful in this myself.
Finally, back to my beginning statement. Do whatever you can to raise money to make more money.
When I needed to make money, since I'm still in school, I bought bulk candy from sams club and sold it in school for 100% mark-up (i.e. a pack of skittles goes for the regular 75 cents, but it only cost me 37.5 per pack when I'd buy in bulk at sams club or costco).
Sell bottled water, pop, etc., just buy it in bulk again at sams club or cost co or some other bulk store of the like.
I still have about 2000 frooties bagged up into bags of 25 in my trunk. I get them down in Detroit in bags of 400 for $3.95. You can sell bags of 25 for 50 cents like me, or if you can find a more profitable frootie/price ratio (i.e. maybe 35 - 40 frooties for a dollar instead), be my guest. It's nice with the 50 cent bags because then even if a kid has change sitting in his pocket, usually he/she is willing to give up a couple coins for 25 frooties.
I would also suggest selling CDs and DVDs, because that can be very profitable.
I'm not going to get into the regular "sell things on eBay" and "mow lawns throughout the neighborhood" because you can find lists of things like that all over the internet.
However, you might want to consider actually getting a job. If you do that, then try convincing your parents to match at least half of what you put in the bank.
If you can get $300 together, consider looking into Site Build It!. I'm currently working on my first website using SBI!, and I can't give you my own opinion until I've given it a good run through for a while, but it comes highly recommended by many people.
If you don't want to put in the work there, keep saving your money. Just remember that the sacrifice of today allows for the opportunities of tomorrow, and that if you save your money now, you'll not only promote very good habits, but you'll also have it to invest in something such as an existing web-business.
Like I said though, the best way to do this would probably be to get a job. Preferably as a waiter or server where you'd get tips, a nice steakhouse. Other places to look for would be banquet halls, nursing homes (I just had an interview to be a dietary aid at a nursing home this previous thursday, I'd be making 8.50 an hour, 9.50 on weekends, plus up to double time on holidays, just to show that they may pay better than fast food restaurants), and maybe some labor work if you have the time and the capability. Theres nothing like tearing off a roof in the scorching sun to put some hair on your chest and some money in your wallet
Other than that, be creative. Good luck though, I'm in the same situation you are in. Me and my friend (aka business associate lol) have a fair bit of money between us, but not enough to do much heavy work. I have some friends willing to invest money if I come up with the right opportunity, but I'm not willing to risk other peoples money unless I am dead sure of the investment, and I'd recommend you'd do the same before you make any decisions when the time comes.
If all else fails, just believe. If you don't have anything to do at the time to make money, invest in yourself. Workout, get healthy, and study up financially-relating topics like marketing, e-commerce, sales (you can always improve your sales pitch), and have faith that you will eventually become successful. You may not get the money you need now, but having faith in yourself and putting in hard work creates the mindset and the mentality you need to unlock new opportunities.
Anyways lol, thats my 2 cents. Sorry about it being so long lol, I kind of just went off since I relate to the topic so closely I guess.