Hi Joanna,you commited a few mistakes,but you did really well translating!
Here I corrected your mistakes,and marked them with asterisks,while correcting the mistakes.
1. Dónde es la fiesta? - Where is the party?
2. Cuántas personas *celebran* en la fiesta? - How many people celebrate at the party?
3. *Que está haciendo Ana en la fiesta? - What Ana is doing at the party?
4. Cuándo empieza la fiesta? - When the party starts?
5. De quién es *el pastel* de cumpleaños? - Whose birthday cake is that?
6. Quién no *vino* a la fiesta? - Who didn't come to the party?
7. Quien esta vistiendo la chaqueta azul? -Who is wearing wearing the blue jacket?
8. Porque Luisa esta tomando fotos? -Why Luisa is making photos?
As they have told you,it is important that you use the sign "¿" at the beginning of a question,that's the grammatical rule,although a lot of people in informal contexts don't use it,so if you didn't use it you'd be well understood still,but if you're using it in a formal situation,it is important that you use it!
If you have any more doubts,you're free to contact me,By the way,I'm a tutor here on italki,and I'm looking for new students,so I invite you to check my profile! You're free to contact me!