Garam masala is a blend of spices, toasted and ground, from India. The spices used are mainly cumin; coriander, cloves, cardamom seeds scraped from the pods once toasted and cinnamon; but there are many variations which can include turmeric, asafoetida and other spices.
It is easy to prepare your own mix at home following the recipe: Garam masala.
To make garam masala in a hurry, combine 1 pinch ground cumin, 1 pinch ground coriander, 1 pinch ground balck pepper, 1 pinch ground cardamom, 1 pinch ground cinnamon, 1 pinch ground cloves, 1 pinch ground turmeric.
If you don't have any, substitute with medium curry powder.