You need to talk to your mom or dad or other responsible adult in your life. Or go to a doctor. And please stop having sex until you have learned the basics of fertility awareness. I know someone will come along and give you excellent information, but I just wanted to urge you to turn to someone in real life rather than strangers on the internet.
Condoms are not 100% effective in preventing pregnancy, but neither is any other method of birth control.
First thing, get a pregnancy test. If you're still uneasy after that, please make an appointment with your doctor and talk to them about your fears and options. There are always options. If it all comes to naught, I would suggest taking a course or reading books about pregnancy prevention. Talk to a sexual health counsellor or professional if you have any other questions or concerns. But talk to someone you trust about this.
Two periods in a month is generally not a sign of pregnancy – the absence of menstruation is more commonly associated with pregnancy rather than an increased frequency. I would recommend that you continue to learn more about your body, as you are still undergoing many changes. A “typical” cycle is 28 days, therefore, there will be times when you could, quite normally, expect to have two menstrual periods in the same month. Also bear in mind that Day 1 of your cycle is the very first day of bleeding, so this is counted from the first day of bleeding to the last day that you are not bleeding – so if your last cycle began toward the end of the month of June, but extended into July, your period may not be coming with a greater frequency than is to be expected. Your periods will normally fluctuate a bit in the first few years as your body is changing, so if you have not had your period for very long, previously, this is quite common. And it is common for other changes that affect your hormone levels, such as becoming sexually active, to have some type of effect on your periods.
If your periods truly are coming closer together, I would suggest speaking with your mother and perhaps scheduling a visit with an OB/GYN to rule out any possible problems and to seek treatment, if necessary.
If an issue is found, one way an OB/GYN may work to help with your menstrual cycles is to prescribe the birth control pill. If this is suggested in your instance, I would research to see how I felt about all of the possible side effects, as they can help in some areas, but like any medication, there are both risks and benefits. Obviously you should talk to a doctor about taking the lowest possible dose of hormones that you can in order to regulate your cycles. And even if you are on the pill, I would highly recommend that you continue to use condoms, as the pill does not protect against STDs.