The OP has long gone, but for anyone else who might be thinking of their first time and wondering if it will hurt...
Yes, if you go wham bam thank you man, it could hurt. The ass is designed to clamp shut when it's not being used as an exit. It needs to be nurtured and trained to take a cock.
It helps to practice, first with fingers and then with toys, starting smaller and working up to cock size. A good quality lube is a must. Ideally this will be from an adult store, and water based for toys, or if you need it to be from around the house, coconut oil. Get yourself comfortable where you won't be disturbed, or in the shower and take your time.
I've found the process of cleaning (douching) helps to relax and dilate my ass. I prepare for anal sex with a good clean, and then over the toilet, dip my hole with a
dildo to flush out any remaining water and as a final check on cleanliness. Don't douche too deep or over-fill. Your plumbing turns a corner about 6-7" in. Up around the corner is tomorrow's poop (the sigmoid colon if you need to google it). If you get water up around your S-bend you'll be flushing for ages. Leave tomorrow's poop for tomorrow and clean out your rectum.
I've found a good technique when taking a cock or a cock-sized
dildo is to push gently (as though taking a dump) which tells your ass to relax and dilate to take it. You have two muscles that clamp shut, called sphincters. The outer sphincter is the one you can consciously squeeze and relax. The inner one is an inch or two in, and is involuntary. When the
dildo or cock head is in, it meets a slight resistance. This is your inner sphincter saying "no". Relax, breathe and push out as you or your top presses, not forces, just presses to allow you to dilate and take it. Once it slides in past the sphincters, the job's done! Your ass will envelop the cock and can slide right up to the balls in heavenly connection.
It's good for your first time to be with an understanding and preferably experienced top that you're comfortable communicating with. Don't take cues from porn, where the guys are experienced and have loosened up before the shoot. He can't just shove it in. First time anal sex should not hurt, and need not hurt if you're prepared and the top goes at your pace and checks in with you.
More info is here on understanding your anatomy and links with more advice:
https://forums.justusboys.com/threads/all-about-ass.272148/