It sounds like you have a very interesting project idea! May I ask what you will be testing?
Science Buddies has a lot of information regarding tips and techniques for microbiology projects: http://www.sciencebuddies.org/science-f ... ques.shtml
. Make sure to read the safety guide first! http://www.sciencebuddies.org/science-f ... fety.shtml
You should be able to use a swab to stain a petri dish. Here is some information about one way to inoculate your plates: http://www.sciencebuddies.org/science-f ... tion.shtml
1) Here is a site describing the most common types of bacteria in the human mouth: http://www.livestrong.com/article/16130 ... man-mouth/
When you do your experiment, you will probably not be able to tell which types you have, although some of the colonies may look different from one another. What you will be able to measure is the number of colonies.
2) You should be able to use premade agar. Here is some more information about agar: http://www.sciencebuddies.org/science-f ... Agar.shtml
3) You should not have to stain your bacteria to see them. Most colonies grow very quickly and are visible to the human eye.
Here is some information about interpreting plates: http://www.sciencebuddies.org/science-f ... ates.shtml
If you end up with a "lawn" of bacteria (the plate totally covered in colonies), it may be hard to quantify. In that case, you may need to dilute the samples before plating them, so you end up with distinct colonies that you can count.
Here is a similar thread from a few years ago, which may answer some of your questions: viewtopic.php?f=3&t=2936
Here is a Science Buddies project that is related to yours. Check out the procedure to help you with your experimental design: http://www.sciencebuddies.org/science-f ... SW#summary
I hope this helps. Please post again (in this same thread) if you have more questions.