In reality, if you stopped to think what you'd say, you'd never become fluent. The key to learning ANY language is always immersion, either by spending time in the country where the language is spoken, or watching films, singing songs or being surrounded by people speaking only the target language. In case none of the above is possible, you can always learn the set phrases, which will help you become fluent.
Learning grammar is always a "reverse engineering" process - and it should stay that way. That's why I will always publish one "Serbian phrases video lesson" based on a situation covered in my Serbian phrases e-book on Sundays. Lots of my students told me it was highly useful and practical, and I'm sure you'll find it the same!
Pozdravi na srpskom (Greetings in Serbian)