read this and practice practice practice:D :D

http://www.sexinfo101.com/pm_fellatio.shtml

Similar problem here. Place your thumb lightly of the bottom base of his penis. When he starts to ejaculate, you will feel it. Then press the back of your tongue against your hard palate firmly. This will protect the gag reflex. That first shot is a surprise and you will likely have trouble handling it. You will know what to expect thereafter and this helps immensely.

...and, in addition:

Raise your tongue so the tip touches the roof of your mouth. When he ejaculates, the semen will squirt agains the bottom surface of the tongue and not go further back into your mouth. Keeping it from flying back and touching the uvula will prevent you from gagging.

That would not be in addition; that is an alternative. I prefer to press the back of my tongue against the hard palate.

If you have issues with your gag reflex its likely that your super super anxious about what your doing which obviously as a firsttimer its expected. My advice is to completely relax and tell your guy that you've never given head before and ask for feedback on how your doing. If you don't want to take him in your mouth very far use one or both (depending on how big they are) of your hands to stroke his shaft and work with just his head in your mouth. As for the cum, if you aren't comfortable with the idea of swallowing you can just pull it out of your mouth and let it shoot else where or you could just hold it in your mouth and spit it out afterwards. Don't worry we all had this problem at some point in time!!:D ;)

Congratulations on being successful at giving head your first time and being able to swallow. There are many women that have a hard time giving head, let alone swallowing. Your boyfriend is truely a lucky man. It should be easier the more you do it. How did your boyfriend react?