In dire need of blowjob advice

So my boyfriend and I have been together for about 6 months now and he practically begs me to cum in my mouth, but I cannot deal with cum in my mouth. I gag if even the littlest bit gets in. I've only finished a guy off with my mouth once before but I really want to be able to do it for my boyfriend. Does anyone know how to get used to it? Or also if you "spit" where do you spit it? (I'm 19 just very inexperienced when it come to BJs)