There are two ways you could say that. The most literally way to read that would be:
"son las once y quareinta y cinco minutos " which is the equivalent of "it's eleven forty-five". However, this is never used when people talk or in books, but you can find it perhaps in technical products like alarms or radios or even train stations.
So, the most correct way to say it would be this:
"son las doces menos cuarto" which is the translation of "it's a quarter to twelve".
Here's a pretty good lesson that explains how to tell the time in Spanish and its different ways, especially when it changes between "es la una" or "son las dos" and so on.
Hope this helps!