I have to do a happy meal box cake I figured out how to do the bottom but how do I make the top stand up can I achieve that with fondant or gumpaste? and how do I do it?
Use gumpaste - get a real box (if you are doing a life size cake), cut the bits of card off the top and use them as templates to cut your gumpastes using a scalpel/sharp knife. Let them dry ahead of time and they will be hard. Attach to your cake using royal icing.
Can I use Cardboard and cover it with fondant? How much in advace will I have to do it?
I wouldnt. A kid might eat it and it would look much more clean if you just rolled out a piece of fondant or gumpaste and used a exacto knife and trace it. Put toothpicks in the end so you can stick it into the box. If using gumpaste a day or so, for fondant atleast 2. Try not to make it too thick ifyou need it to dry fast