If you are sexually active (and sometimes even if you're not) you can get an STD. Here are the symptoms that should concern you.
You may have an STD if you have any of the following symptoms:
burning sensation while urinating
unusual discharge or pain within one to three weeks of last sexual encounter
smelly, foamy, yellowish-green vaginal or penile discharge
abnormal vaginal bleeding
genital area discomfort
fever or rash on skin or genitals
soft, itchy warts in and around the vagina, penis, and anus
soft, cauliflower-like bumps on genital areas
cold sores and fever blisters on the mouth (these can be spread to the genitals during oral sex or if a hand touches the genitals after touching the sores on the mouth)
swollen lymph glands with or without sore throat and fever
red bumps that turn into painful blisters or sores on the vagina, penis, buttocks, or thighs (sometimes on other parts of the body as well)
sores on the genitals or mouth that disappear but are followed with a rash on the palms of the hands and soles of the feet within 3 weeks
severe fatigue, aching, nausea and vomiting, loss of appetite, darkening or urine, or abdominal tenderness
yellowing of the skin and whites of the eyes (called jaundice), and darkening of the urine followed by fever and cold spells
unexplained weight loss, flu-like symptoms, diarrhea, fatigue, persistent fevers,
night sweats, headaches, mental disorders, and/or severe or recurring vaginal yeast infections
If any of the above are accompanied by fever, itchiness in any part of the body, headaches, dizziness, nausea or a change in appetite you must seek immediate help. If you suspect that you may have an STD based on the list above, a visit to the doctor is in order.
If you have had sex with an unfamiliar partner, if you have had sex with somebody new, if you think you may have had sex but aren't 100% sure, and you see any of these symptoms you should get to a doctor ASAP.
If you were raped or molested you should go to a doctor immediately, even without symptoms.
If you have had unprotected sex for any reason and you have any of these symptoms, do not hesitate, get thy self to a doctor!
Not all STDs are spread through intercourse, many can be spread through simple touch. If you have oral sex with a person who has cold sores you could develop lesions on your genitals which could progress to genital herpes. If somebody has open sores (not wounds caused by injuries, but sores) on any part of their body do not have sexual contact with them until the sores have cleared up.
Not all STDs have symptoms, many are symptom-less. Guys have symptoms less often than girls, but they still get STDs just as often. Because of this fewer guys seek treatment, and many are carriers without even knowing it. Since many sexually active girls have regular gynecological exams, STDs in females are more likely to be caught early and/or discussed with a doctor. Just because you have no symptoms doesn't mean you are disease free, you can even spread diseases to people when you yourself have had no symptoms. The person who has symptoms first is not always the person who had the STD first. An STD does not have to be flaring up to be contagious.
Some STDs are treatable with antibiotics, others stay with you for life, be careful and practice safer sex.