I usually pay off my friend or mom to get them for me (yeah I'm mature) at the store- I wouldn't get 25 cent ones get Tampax or something. How to put it in... Sit on the toilet with your legs apart. Use one hand to spread your vaginal lips apart, while holding the tampon with the other. Insert it into your vagina, angling toward your tailbone (not UP toward your stomach) as far as your fingers will allow, then press the plunger till the tampon leaves the applicator. Remove the applicator; the strings attached to the tampon should be outside of your body. When it's time to change (every 4-6 hours) just gently pull down on the strings in a steady motion. Insert a new tampon and you're good to go. (from http://answers.yahoo.com/question/index?qid=20061215123329AALuyCD)

Hey, don't worry about buying them, it's something every girl has to do at one time or another. Since you are young, the vended ones may cause some discomfort, so I would suggest going with the slims that most brands have. Talk a friend or your mom into going with you (for the longest time I had to go with my GRANDPA!!!) and try just looking around at which kind you would like to try. There will also be a small brochure in any box that can give you any and all instructions and any other information you need to know. Since every body is different, your flow may stop in water, or it may keep going. I would suggest getting a tampon in any case. Better safe than sorry.