Field check and validation

Field check and validation

Hello people,
I'm new to this Zoho Creator and I'm developing my very first on line form.
You can find it embedded here: http://www.suezo.it/iscrizioni-on-line (sorry ... it's in Italian!)

This may be a silly question, but since I'm new to this it's causing me some trouble!
I added a few "number" fields (quantity of tickets) but I need to check and validate numbers.
Example: one field needs to be "20 or more" so I would like not to the form to be submitted if that field is 19 or less.
One other field needs to be either 4, 8, 12 ore 16. Same thing: I need to inform the client it's wrong to put any other number.

Is that possible? How can I do it?
I tried to go through that field properties / actions script thing but it sounds a bit complicated to me so I couldn't make it...
Any help?

Thank you so much in advance!
Gino