How to create Formula Field on Opportunity in Salesforce?

Hi all- been a while since I constructed one of these and wanted to get some help- On the opp object, looking to create a formula field based on 2 fields- Amount & License_Period_months__c ( A number field type).

Here's what I'm trying to accomplish:

If

License_Period_months__c = 12,  Amount

If

License_Period_months__c = 24,  Amount * 2 * .15

If

License_Period_months__c = 36,  Amount * 3 * .30

I'm screwing up the IF statements here and getting errors. Can I get some advice on how you'd construct this to get the right amount? And would the field formula type be Currency? thanks!