It is hard to give a definitive curry recipe as each family has its own way of making them and then there are so many regional variations too. The creamy curries tend to be from northern (more cold) regions of India. My family is from a part of India that does not normally have creamy curries but it is predominantly vegetarian so my advice is based on this.
If any of you watched come dine with me this week and heard Paz talk about making a curry - well, that is how I was taught too. Heat some oil in a pan add mustard seeds, cumin seeds, aesofetida and curry leaves - put a lid of the pan as the mustard seeds will pop as they heat - this should be on a medium heat. Add onions and cook until softened - again with the lid on as this prevents the onions burning but remember to stir. with vegetarian curries, we tend to add the veg and then the tomatoes at the same time then add any other spices - normally this is a quarter teaspoon on turmeric, chilli powder, ground cumin, ground coriander seeds and salt. In any even, add tomatoes or passata after the onions have softened as adding tomatoes before the onions are softened dramatically slows them softening.
Some of the spices such as the aesofetida and turmeric are to aid digestion so, i would never omit them even though they don't add much to the overall flavour.
If I had to teach someone to cook a curry, I would start with a quick potato curry. Peel and chop a couple of large waxy potatoes (desiree) into 'chips' and dice an onion which is optional. Heat a couple of tablespoons of oil in a pan, add a teaspoon on mustard seeds and a teaspoon of cumin seeds and a generous pinch of aesofetida - after the mustard seeds have popped, add the onions, if using, and potatoes and stir to coat the onions and potatoes with oil, add, salt, fresh green chilli and ginger to taste and a quarter teaspoon of turmeric, put the lid on and leave to cook on a lowish heat. once potatoes and onions are cooked through, taste and adjust seasonong if necessary and serve sprinkled with chopped fresh coriander.
Another verion of the potato curry would be to cut the potatoes into larger pieces say one inch cubes, add tomato passata after the onions have been softened and add a half a cup of water and simmer. For this version, you don't need waxy potatoes as floury ones help thicken the 'sauce'.