What are you testing? If your testing the effectiveness of soap on bacteria (gram negative and gram positive) then you can run a fairly simple experiment.
Produce some agar jelly plates smear them with the gram negative or gram positive bacteria. Something like E.coli (gram negative), and S.Bacillius (gram positive) or any other two. Remember if you don’t use these two to make sure your not using anything with a Beta-lactase ring or other resistant type bacteria. Or you might get complications with your simple experiment. Try and keep the possible anomalies to a minimum.
Now you have your smeared agar jellies. Now in the well add the soap. (If you want to do a more in-depth test you could use different percentage of the soap by simply watering it down. You will need as many plates as tests you wish to do)
Lets pretend you are doing it with the two types of bacteria and 100% soap. Now you added the soap to the wells incubate them for a week. After a week look at the “area of clearance” that is the area around the well with no bacteria growing. Put the plate on some graph paper and crudely work out the area of clearance. Then compare. You might try this with several different soaps at 100%.
If you need any more info let me know.
P.S. for more answers try tagging this under biology and biochemistry as it falls under these categories better. It definitely is a biochemistry question ;)