The shower after your first sparring session probably won’t feel good. The sweat washing down your body will cue you in to dozens of little mat burns that you didn’t notice previously. Generally, these are most common on your elbows and knuckles (especially on the knuckle of your big toe).
It’s a good idea to wash these little cuts out thoroughly and disinfect them. I once knew a guy at a previous club I trained at who got some seriously nasty mat burns on his feet, didn’t disinfect them, and ended up in the hospital on antibiotics for a few weeks with an IV in his arm. His was a pretty extreme example, but nevertheless a good reminder to pay close attention to general hygiene both before and after training.
Looking in the mirror, you’ll also probably notice a handful of little bruises (a.k.a. smurf bites), especially on the inside of your biceps. This is probably from getting pinched beneath your opponent’s knees when mounted (which is a position you’ll spend a lot of time in at first). Amazingly though, your body will toughen up over time. Little by little, you’ll stop getting bruised. Your feet will almost never get mat burn after a month of training. As you progress, your neck that always felt tight after getting choked will loosen up, and you’ll stop feeling like a decrepit zombie after training.
Don’t get discouraged – think of this initial adjustment period as a rite of passage. Once you make it through, and you will, you’ve passed your first test (of many) on your journey in Brazilian Jiu Jitsu.