You might catch very soon, it may take longer than you think. Just 18, what's the hurry?
A rock solid relationship is the best foundation. You might think it will just be you three but have as much family/friend support you can get. Some friends will drift away before you reach nine months. If you want to prove how adult you are, please don't rush into parenthood. Escaping a poor upbringing, edgy family background? travel instead.
Sex during later stages of pregnancy and for up to months after the birth might not appeal. How does your boyfriend feel about celibacy?
Remember when you tell everyone, some will be thrilled. But some will tell you you're crazy and random strangers will inform you that you are eating and drinking wrong, or you are so young it is immoral, or worse ask leeringly were you careless or drunk? Later they'll push past, grab the last seat on public transport, let heavy doors shut in your face and pat your bump without asking. And at least four will make you cry with horror stories about labour and birth.
That newborn requires 24 hour a day care. Set your alarm for 3 am one night this week. See how awake you feel. Get up and walk around for ten minutes. Go back to bed but set your alarm for 4:45 am. Do something intricate with only a low wattage light on. Then set your alarm for 6 am. Does your partner even wake up?
Eat well, sleep well, look after yourself. Are you employed? Save as much money as you can. Babies cost. That said, they don't need brand new everything. If you know someone with a baby ask if you can practise holding, picking up, soothing.
When you get a positive result, don't think your new life starts when s/he arrives. That begins from that moment you find out.