Hi there,
It sounds like you need help in narrowing down your project.
pH is easy because you can buy pH strips online. They're safe, cheap, and easy to use - just dip the paper in the water. The only downside is that the strips are often not highly sensitive, so you might have trouble distinguishing between, say, river water at pH 6.7 versus river water at pH 6.2. For more detailed analysis, you could ask a chemistry teacher about doing a titration.
Testing for "pollution" is different. How are you defining pollution? There are so many different ways to define how "polluted" water is, so you might want to do some background research and figure out which way you want to test in your project. For instance, you could see how "clear" the water is, with the assumption that more polluted water is more murky. (Something like spectrophotometry would be one way you test "clarity" of water.) You could test for certain chemicals, like nitrates, phosphates, and/or heavy metals, and there are specific chemical ways to do that.
Testing for bacteria would probably require you to streak the bacteria on an agar plate and monitor growth to see how much bacteria grows how quickly from the water inoculation. We have detailed resources on how to streak plates here:
https://www.sciencebuddies.org/science- ... plates.pdf
In general I recommend you do more background research about what water pollution means, and what kinds of ways you can use to test it. I'm sure you could also reach out to local water treatment plants or water testing labs/companies to learn what they do and how they do it.