Here are some examples:
* To danie jest ostre. (This dish is spicy.)
* Lubisz ostre jedzenie? (Do you like spicy food?)
* Dodaj trochę pieprzu, żeby było ostrzejsze. (Add some pepper to make it spicier.)
You can also use pikantny which is a bit more formal and literary.
* To jest pikantny sos. (This is a spicy sauce.)
Uwaga: Be careful, because "ostry" can also mean "sharp" or "pointed" depending on the context.