应该是There will be a party!但个人认为There's going bo be a party比较好点.
There be主要描述某处有某物或某活动,比如树上有鸟,There's a bird in the tree.
而have刚一般用在sb have sth,某人拥有某物,比如我有一本书,I have a book. Have的主语不一定是人,可以理解为有生命（或者从表达意义上来讲）的东西. 比如,Even a dog should have its own home. 就算一条小狗也应该有自己的家.