According to a recent study, 10% of women have been forced into some form of sexual activity aged 16 or younger.*  It is against the law to have sex with someone without their consent (permission).  According to the Respect and Consent fact sheet from the Sexual Assault Resource Centre (SARC) you can not give consent if you are:
   drunk or under the influence of drugs;
   unconscious or asleep;
   feel pressured or threatened; or
   are under the age of 16 (in WA).
You can find the full fact sheet here.
It's your body and you don't have to have sex if you don't want to.  If you have been forced to have sex you can contact SARC on 1800 999 888. Check out their webpage for young people here.
* de Vusser RO, Smith AMA, Rissel CE, Richters J, Grulich AE (2003) Experiences of sexual coercion among a representative sample of Adults. Aust NZ Health.