I'm planning to announce my engagement at a birthday party.
What would happen if you announce your engagement at this birthday party?
My boyfriend is going to become angry at me for making a fool out of him.
It's understandable to be excited about being in a new engagement, but you should wait until both parties are ready to do so. If your partner isn't ready they may be upset that you didn't consult with them first. They may want to call off the engagement in retaliation.
I don't care, I want to tell everybody that we are engaged.
It may end up very badly for you if you surprise your boyfriend this way. For your relationship, and the beginning of your future together, it is better that you agree together on when to tell people about your engagement.