The most common problem that causes a loud rubbing sound is a tire, usually the back tire, rubbing on the frame, and you can usually see exactly where it is rubbing. Brakes can rub too, but they usually are not that loud, so let's assume it is the tire. This happens when the rear axle wasn't tight enough, and when you pedal hard or hit a pothole, it can make the axle pivot in the slots it fits in, and this lets the tire to rub on the frame, usually on the front part of the rear tire.
Solution: For this kind of work, I usually flip the bike upside down on an old piece of carpet, etc. so it is sitting on the seat and handlebars with the front wheel pointing straight ahead. There are two common methods to secure the axle:
1. Two pretty good sized nuts, one on each side. Find a wrench that fits just right. I prefer a socket, box end or open end wrench, one for each side. On metric nuts, it will often be 14mm or 15mm, sometimes bigger. American sizes are usually in the 9/16 - 5/8 - 11/16" range. I strongly discourage you from using any kind of pliers or even an adjustable (crescent) wrench. You have to tighten these babies pretty tight, and you can easily burr the corners off your nuts with adjustable tools, believe me I've done it. The tricky part is you have to do three things at once. First, you have to keep the front part of the tire evenly spaced between the two sides of the frame. Next, you have to slide both sides of the axle back in their slots until the chain has the proper tension. If you have a ten-speed style bike, the derailler mechanism will adjust the tension automatically for you, so slide the axle all the way back until the side with the gears is against the back of its slot, and let the other side move forward or back as needed for the tire to be centered between the frame. Finally, while you are keeping things lined up - a patient friend who is willing to help makes this much easier, just have them hold the tire so it is evenly spaced between the frame, and then you have to tighten the nuts. If you don't have a ten-speed style gear changer on the back tire, you have to take up most of the slack in the chain yourself and hold it tight until you get those nuts tight enough to keep the axle from slipping. Don't be surprised if you have to loosen up the nuts and do it again - on a single speed bike you should have about 1/4" to 1/2" of flex in the middle of the chain, halfway between the front and rear sprockets. Too tight, and it can wear out your bearings or chain well before their time. Too loose, and your chain will fall off at the worst possible moment, and you will have to do this process all over again, after you push your bike back home. Tighten a little on each side until things get snug, and if the tire is still centered between the frame, do both sides again, harder now (grunt a little this time, it helps) and you should be good to go. Remember, you are not trying to strip the axle threads, or break anything, but you do have to get it tight enough so it won't slip on you again.
2. Oh, yeah, there is another common method you find pretty often on ten-speed style bikes, the quick release.
This is an assembly that consists of a lever built onto the axle nut, and the lever is only on one side. You don't use a wrench on the quick release, but they are a little tricky until you understand how they work. As you pull the lever away from the frame, a cam inside loosens the axle, and as you push the lever toward the frame, it tightens. When the lever is in the loose position, you can also spin the nut on the axle tighter or looser (careful, it doesn't take much, and clockwise should be tighter on most bikes). Tightening or loosening the nut part does most of travel, and the lever does the last little bit. The lever is short, usually only 2-3 inches, so if you don't have to push pretty hard on the lever, the nut is probably too loose, and you need to loosen the lever and rotate the nut part clockwise a little bit, until it feels like the axle is getting really good and tight just about the time the lever gets close to the frame. This can also take 2-3 tries of loosening the lever, tightening or loosening the nut, and retightening the lever again, until it feels good and tight, and of course, you have to check your tire alignment one more time to make sure it is still nicely centered between the frame of the bike. Turn the wheel by hand a few turns to make sure it doesn't rub on the frame. If everything is tight, and your tire is still centered, you're ready for a test ride. Just up and down the driveway to start with, and make sure your brakes are okay. Then you can go a little farther, and pedal a little harder. Hopefully the axle will be nice and solid, and you can say, "Good Job! I fixed it myself, on Fixya!"