Learn Finnish: Minulla on yksitoista tai kaksitoista euroa.

Finnish doesn't have a direct verb for "to have". Instead, it uses a structure that literally means "at somebody is". Minulla means "at me", and on is "is". So, Minulla on translates to "I have".

What does the "-toista" ending in yksitoista and kaksitoista mean?

It translates to "of the second". To make the numbers 11 through 19, Finnish takes the base number (yksi for one, kaksi for two) and adds -toista. You can think of it as "one of the second ten".

Why is the word for euro written as euroa and not just euro?

In Finnish, any number greater than one must be followed by a noun in the partitive case. To make the partitive form of euro, you just add an -a to the end. Since eleven and twelve are bigger than one, they force the word into the euroa form.

Could I use "vai" instead of "tai" for the word "or" in this sentence?

No, you must use tai here. Finnish has two words for "or". Tai is used in regular statements like this one. Vai is only used in questions where you are asking the listener to choose between two options.