Blowjob help

My boyfriend is really well-endowed so it's hard to fit even the tip of his penis in my mouth. I've done it before but my teeth keep on scraping against it and I'm not sure how to avoid that. It's just difficult since he's so big. Also, he's given me a few pointers on how to give a good blowjob but I still struggle with it (I was a virgin when I met him and he's sexually experienced). If you have any tips please comment with them below.