Are e coli bacterial colonies countable on an agar plate?

They look like little white circles.  

If there are between about 20 and 250 colonies they are considered countable.  If there are too many you should have diluted before plating.

When you add them to the agar, add a small volume then spread with a sterile bent rod.  Spread evenly, then wait for the water to be absorbed before inverting and incubating.

If the colony grows from a single cell, it will look like a round, slightly raised thing, off-white or light beige color. Depening on how long it's been growing, it will be maybe a few millimeters in diameter.

You can easily count E. coli colonies on a plate, as long as there aren't so many that they overlap each other. A good number for counting is 100-300 per 10 cm diameter plate.

I don't know how you plan to do your antibacterial soap test, but when you plate the bacteria, you should have them suspended in liquid at a concentration of ~ 1000 - 3000 cells/mL. You probably won't know the exact concentration, so the best thing is to start with a higher concentration and make a series of 10-fold dilutions.

Note that the liquid you suspend the bacteria in should ideally be something that they 'like' such as liquid growth medium. If you put them in pure water, the absence of salts may cause some of the bacteria to die independent of any effect of antibacterial soap.

Without certain equipment, it can be difficult to know how many cells per mL you have to start with. One thing that might help you: hold up the liquid and swirl it around. If you can just barely see a little cloudiness swirling around, you probably have about 10,000,000 cells/mL. Use that as a starting point to decide how much to dilute, but be sure to use dilutions that you think may be too high and too low, as well as one that you think is right, to be sure that one is countable.

Next, put 0.1 mL of each dilution on the surface of one agar plate. (Or, use several agar plates for each dilution, if you want more robust data). Next, take a sterile rod and spread the liquid around evenly on the agar. Your science teacher may have rods designed for this. If not, try to find a bent glass rod, or something else that's nonporous. You want something that won't absorb liquid or have a lot of cracks that the bacteria could get stuck in while spreading them. Sterilize it by soaking in isopropyl alcohol for a few minutes. Then take the rod out and hold it in the air until the alcohol evaporates. Don't set it down again until after you use it to spread the bacteria! Otherwise it may get contaminated with some other bacteria and may mess up your experiment.

Cover the plate and wait 5-10 minutes to let the liquid get absorbed. Then turn the plates (with covers on) upside down to incubate. Best incubation temp is 37C, but room temp will be OK if you don't have a 37C incubator. (The reason you incubate plates upside down is that sometimes condensation will form inside the plate. If the plate is upright, condensation can form on the lid and then drop onto the agar below. That will cause any colonies underneath to get smeared around and be hard to count.)

At 37C, you should see colonies within 1-2 days. At room temp, you may need to wait 3-5 days.

Good luck!