You have to flow enough, else you will be putting the tampon into your 'dry' vagina. This is both uncomfortable/painful and unhealthy as it will dry out your vaginal walls. Normally you will not flow enough during early puberty. Removing a too dry tampon is also extremely uncomfortable and painful to remove!
Buy the smallest lightest tampon for example Teen, Lights, Mini or Slender. If you have a choice in those sizes, take a plastic applicator.
Your hymen might also prevent you from using tampons.
You have to make sure you put it in deep enough so that you do not feel it.
Read the tips in the tampon booklet to find a comfortable position.
Congratulations with getting your first period. You are growing up and becoming a woman!!