Listen, if this is your first time then tell her to not push you hard, try having her kick you continuously with soft kicks, then after a few soft kicks she should kick you harder once in a while, i suggest she shouldn't do it hard as it's your first time, ussually she should kick you hard only when you can take it, see that diminishes the risk of you getting injured, you both can try kicks from her standing in front of you, if you want you can try it with you taking the kicks when you stand on your knees, see that hurts more
or you could try taking them from behind so you don't see the kicks them being kind of a surprise, when you are more advanced and have done it a couple of times you cand try it with your hands tied behind your back with rope or cuffs, i suggest cuffs but always be careful in chosing a responsible female partner that is willing to let you go when you have had enough, always keep it safe, set up a safe word and when you say it she sould stop or just tell her when you have had enough and then you stop. As for the pain, you can't relly compare it, pain is just pain, soft kicks hurt less and hard kicks obviously hurt more, you should always tell her if she goes too hard so that it remains satisfactory for you but sometimes that just cuts out all the fun because i guess sometimes you just want to be kicked hard
, knees hurt badly, sometimes a hard knee hurts more than a good kick if she gets both of your balls, you just hear her knee slamming quicky against your balls and that is it for you, you're into a lot of pain.
Well i hope that helped and remember always keep it safe, you don't want to be having to visit the urologist on a weekly basis. Cheers