Garam Masala is one of the most common Indian spice mixture. “Garam” means “hot”, masala actually “mixture”. If You do not have it ready to buy, here is a basic way to substitute it.
2 tbsp ground coriander
2 tsp ground cumin
1 1/2 tsp crushed black pepper,
1 tbsp ground cinnamon
1 tbsp ground cloves
Just mix all the ground spices together and follow the recipe.
Authentically all the spice are dry roasted first whole and then grounded. Just heat it through in the oil and onions before adding extra ingredients. You will have similar effect. All of those spices have fat soluble aroma compounds, so it will work well. Just watch out! Spices are easily burnt, that will spoil any dish.