first off relax, make sure the time is right for both of you, if he is big and you are new it always adds to the equation. take things slow and just enjoy being with each other and exploring each others body. if u r finding it hard then try some lub ky jelly is good as a basic. don't expect too much either, the more you do it the easier it will get for both of you. the guy i'm seeing now is large and sometimes when we start i clam up there a little when he wants to enter me, only brief, but once he's in etc its a very pleasant experience.
lots of foreplay first, try fingers, starting with none then adding more if its pleasant for you, when u r ready u can slowly ease it in, dont rush things and have fun