Some common symptoms of STD's:
Unusual discharge (watery or puss-like) from the penis, vagina or anus.
Burning feeling or pain when urinating.
Pelvic discomfort (girls) or testicular pain (guys).
Itching in the genital area.
Blisters, sores, ulcers, warts on or around the penis, vulva or anus.
A wound or blister on or around your lips.
Swollen lymph nodes in the groin

If you have any of these symptoms it could be something more innocent than a STD, like a bladder infection. But to make sure you need a physical check-up.