Here's the list of four Indian spices I can't live without:
Garlic - not really known as Indian spice but used in many dishes; if you're cooking Hare Krishna recipes but are not into Hare Krishna beliefs I recommend replacing asafoetida with garlic. And I recommend Hare Krishna recipes (unlike their beliefs), they are usually extremely detailed since they can't taste the dish before they offer it to Krishna.
Turmeric powder - this is the spice that gives color to curry, and it likes to give color to anything it touches, especially wood and plastic. If you spill it, it'll stick more if it's wet.
Caraway powder - most recipes use cumin seeds, but in my opinion powder fits better with most dishes, and I like caraway more because it has similar but more intense taste.
Chilli powder - again, I like powder because it allows for more precise dosage.
Obtain these and you can cook many dishes. And then there are non-essential but more often used spices:
Coriander - seeds and powder are interchangeable; but coriander leaves can be replaced with parsley
Cinnamon - powder or sticks
Ginger - fresh ginger is better than powder, but it doesn't last
Garam masala - a combination of spices, usually mixed in the dish before serving
There are a lot more spices in Indian cuisine, but for most dishes this is more than enough. Rincewind's remark is valid - I prefer most of the spices in the list in the form of a powder, but they last longer in the seed form, and taste better if they're freshly ground, but the difference is not as important, especially if you're still experimenting.
Most of the Indian dishes use pretty ordinary ingredients - it depends on the recipe itself. Once you cook your first dishes, you can try making ghee, paneer or chapatis yourself. But until then, you can replace ghee with butter, and paneer with tofu, and skip the chapatis.