Depends on your tastebuds! I understand that there are a million, closely guarded ways to make 'curry'. Most of the stuff we get in England is a long way from the dishes served in Bangladesh and India.
Anyway, my method is: Toast some spices first - especially cumin seeds and shelled cardamon pods - in a dry frying pan, then grind them together in a pestle and mortar. Other spices are turmeric, paprika, and fenugreek. You can also use herbs like coriander, methi and mint - but add those only in the last few minutes of the cooking so they don't lose their flavour. The best way to find out what you like is to experiment - if you've got an Asian supemarket near you, just buy bags of stuff and try them out!
The basic method is to fry onions and garlic in ghee (clarified butter) and add seeded and skinned tomatoes, tomatoes, chillis and your ground spices. If it gets a little dry, or you're planning to cook a red meat such as lamb, then I'd also add some stock to stop it drying out completely. You can also add veg - mushrooms and peppers work well.
For me, the best result is to then slow cook lamb in the sauce - even over the matter of a few hours. As you cook it, just keep tasting and season to your taste. If you want to take the heat out of the sauce, or just prefer something a little sweeter, you can spoon in some double cream at the end. Some people use yoghurt, but I find that it sometimes curdles (a problem I've yet to crack!) Coconut milk is a good substitute too.
Finally, after you've taken it off the hear, stir through some Garam Masala - which is a mellow mix of ground spices that really finishes the flavour off. You can make it yourself, but the bought stuff is actually pretty good - Schwartz make it, for example.
What spices and herbs you use is really down to your personal choice. For me, anything with masses of coriander or methi works a treat.