sodas and bottled drinks have probably the best Return on Investment of any legal business outside of pharma and alcohol.
You can start a bottling plan with about $500 USD in equipment and materials, then build up as income rises.
1) make your mix. The sky is the limit here, Sugared stuff. Caffeinated stuff. "Coolers", which is a mix of wine with sugary stuff. Diet drinks (actually cheaper to produce, and easier since there's less chance they will ferment).
2) Carbonate: The idea is that you fill your bottle with liquid, screw on the cap so the airstone is inside the liquid, and add CO2 carefully, that's what you want the regulator for, I have worked with 60 pound per inch pressure. You want bubbles as small as possible.
DISCLAIMER if anything blows up, it is 100% your responsibility, this is the way things have worked for me, they may not work for you, you're on your own!
Counterintuitivelly, shacking the bottle at this stage has helped me to have more of the CO2 stay in the liquid. Test what works for you. I HIGHLY recommend you work with low pressures until you really can get comfortable. DO NOT do this in half filled bottles or worse empty ones, that increases risks of explosion, always work with bottles that are at least 80% full of liquid.
Then close the CO2 valve, release and remove the cap with tubing and airstone carefully, and screw on a tamper-proof cap. Label, sell.
A safer carbonation procedure is to simply bubble inside a vat, for a couple hours at ambient pressure. This uses much more CO2, of course. Transfer into a bottle using a funnel or jug, cap, sell.
Because of the huge Return on Investment of making and selling sodas, your problems might not be how good your stuff is, but whether you're stepping in someone's turf
Far far away and many years ago, dad and I started bottling a gin ale-type drink somewhere South. Honest and stupid that we were, we sought first official permit. We went through the very straightforward paperwork, but an "insider" told us, we would never receive a market permit. Turned out there were other interests at play... (there's a big bottling plant where we used to live)
Contamination If you get some yeast on your sugary stuff, you may have fermentation happen. If you are lucky people will take it as champagne (hey, you might do it on purpose), but it won't help your image. Hygiene is a must, and until you can afford automated aseptic equipment, boil what can be boiled. If you manage to make beer this way, then you are on to real real money
EXPLOSION RISK While I have gone up to 120 psi pressure (pounds per inch), that was with Developed World bottles. Bottles manufactured in your country might be thinner, have irregular material, whatever, and they might blow up. YOU ARE ON YOUR OWN and should find what are your safe limits. When bottling I have never gone above 60 psi.
Did I say you can blow plastic shards into an eye or both, or other sensitive parts (people often hold the bottles between their legs when filling them up)? If I didn't, now you're warned.
 Things You will Need
1) A clear commitment to hygiene. If your stuff is not safe, it will make people sick, and if you're not clear about this, if you don't care about making people sick, you probably could go into other lines of business that make even more money.
2) Safe water. If you have access to it, you're in business, you could start simply by bottling safe water! If your community doesn't provide you with it, selling safe water might be a great business, but you have to figure out how to get that safe water to begin with. Boiling it is a start, it is more expensive per unit than other methods, but has an extremely low cost to begin.
3) generic single-use bottles and tamper-proof caps - suppliers of these are usually a little bit out of the way, but I have found them in different countries. Metal caps are also a possibility, on reusable glass bottles, but hygiene and disinfection can really be complicated for small operations.
4) large vats that you can disinfect and use to prepare your mixes. Again, watch out for contamination. ladles, jugs, funnels. Boil all of this between uses! Also boil the airstone!
5) some edulcorant. One advantage of chemical edulcorants (aspartame, sacharine) is that there is much less risk of fermentation. They can even be cheaper than sugar itself. And you can call your product "diet"! Yet sugar has some nutritional value, and that might be an ethical choice sometimes.
6) some color. Please, please stick to food-grade stuff. I know people sometimes use anilines to color foods in developing countries to make them attractive to kids. I mean, that's poisonous stuff, bad, bad. Depending on your circumstances, you could even make your own food coloring, good old caramel is a basic one, easy. Did'ya now that carmine red is an FD&A approved food dye (E 120), and comes from cochineal bugs? Near La Paz I can get as much cochineal as I care, right off cacti.
7) some flavor. Here's your chance to make history. What everybody does, including the big companies is to boil stuff together. Ginger, cinnamon, whatever, be my guest. You may want to measure carefully what you use, so you can reproduce and fine-tune your flavor batch after batch. Once you have you flavor extract, mix it with measured amounts of color and edulcorant in the water, and you are ready to go! see  below.
Carbonate stuff 1) CO2 gas tank. you will eventually want to buy your own, but try to rent first. They will ask for a deposit, and usually the monthly payment is so low you may want to hold on buying yours for a while. About $400 deposit, but you get it back, monthly rent was about $5, and a full tank (goes a loooong way) was about $15
2) They will have you probably buy your gas regulator. This might be your highest startup single non-refundable expense. I still have mine, was about $200 USD, but you might get lucky and they will also rent it.
3) "bubbler". I made my own out of plastic pipe. I now recommend a fishbowl airstone, see link  below. Your goal is to make bubbles that area as small as possible Putting this together is the tricky part, and some silicon glue will come handy. You want to set things so you have an airstone that goes into the bottle, while the tubing goes through one screw-on cap.
4) tubing, plain plastic. My first was off an hospital-style drip contraption.