The first thing we have to think about is 

                How many people are coming ???

In todays society with invites going out to all friends on the internet it's not alway an exact science more a hit & miss but you can gauge approximate numbers & that will be close maybe 2-3 of each finger food per person because some will eat more & some won't eat at all...

 

Once numbers are established we need to organise                what kind of food we are having...

for a group of people over 10 I'd always go the finger food with bbq option it always goes down

well ....  over 10 with a sitdown dinner party can be quite hazardous to your health unless you have a master chef in the family !!!

 

Once numbers & Menu are decided invites & decorations are the next thing on the list.....

 

One more thing to think about is if it's a surprise party what devilishly unique plan & story are you going to use to get the victim :-) out of the way for the day ....

 

Don't try & do it all by yourself find the few people you can trust & delegate some things out !!!