He asked me to put a finger in his butt

So last night I was hooking up with a guy and he asked to put my finger in his butt. I was really not about it because I’ve never tried anything like that before. And then I halfway tried and couldn’t even reach over and do it. So then we just dropped it all together and continued. But does anyone have any tips for fingering a guys ass. And do I have to cut my nails. Please help.