What do you think if I skip a really good friend's birthday because I would have felt uncomfortable if I went there?
That's a bummer. What will make you uncomfortable at the party?
Because I don't know anyone but his friend and no guys will approach me
Your friend would be disappointed if you're not there, like they weren't important enough. You need to get out of your comfort zone for this day only, for the sake of your friend because you are supposed to celebrate great things like this with them.
Nah, I'm not attending his birthday anymore.
I think your friend really wants you there. It would be rude to not go, and your friend will be thinking about why you are not there during the party. Why don't you invite some of your friends to go with you, so that way you won't feel left out?