Find time to kick the bottom part of a heavy bag at your gym and just do it over and over and over. Don't overdo it, the pain will let you know when to stop for the day. Keep doing this and over time your shins will toughen up.
As for sparring, just tell your partners to go lighter with the kicks for a while. No point in busting up your legs so badly that you can't even train. What's worse is if you hurt your leg in sparring and become gun shy of blocking because of the pain and just try to take it on your leg, then you'll just screw up your leg and develop bad habits.
So in summary, kick the bag, take it easier in sparring for a while, and take care of your legs, it's the only pair you have
