There are three holes down there...the urethra
, which is really small - a tampon will not fit in there, the vagina, and the anus. The tampon goes into your vagina, the bigger hole that's in front of your anus (where you poop). You might want to try using a mirror to look down there to get to know your body and figure it out more easily.
Since you're only 13 and still young, I would recommend buying junior sized tampons...I know you can get them from Tampax and maybe from Playtex as well. In that case, the Playtex ones would probably be the best. Junior sized tampons are skinnier than normal ones, so you will have less difficulty inserting them. OB is a good brand but most don't have the applicator, and for a first time user, it would be best if you got a brand with an applicator (Playtex or Tampax).
They come with detailed instructions inside the box, and the brands have websites with detailed instructions as well. On Youtube, you can even find videos of people demonstrating on their hands how to insert them!
Just remember, it's difficult and frustrating at first for many people, so don't give up! I didn't get the hang of it until I was 18! Good luck :)