These are the requirements to buy a second level spell (i.e. from Fire to Fira):

1.) Be on the Forum for at least a Month.
2.) Have the required amount of money.
3.) You can skip the middle spell and go straight to the advanced spell if you pay half the regular cost in VP Points. So if regular cost was 20 Munny, you'd have to pay 10 VP Points.

Requirements for third level spells:

1.) Be on the Forum for at least 3 Months.
2.) Have required amount of money.
3.) Have at least 15 MP.