A curry is basically a stew which can use almost any main ingredient whether that is poultry, fish, shellfish, meat or vegetables.
You will need spices of your choice...garam massala is a good blend and chillis if you want. It's your taste so they're not essential.
Next, you'll need something to make the sauce with. Greek yoghurt is good or you can use tins of finely chopped tomatoes.
Personally, i use Pataks paste...2 good spoonfulls. There's mild ones and hot ones, it tells you on the jar.
Bung in a wok or saucepan/stockpot along with a chopped onion (not essential) and your choice of meat/fish etc. Stir fry on a fairly high heat for about 5 minutes....the smell will be awesome, but stir continuously.
Then add anything else you want...pineapple, sultanas, apple, mushroom, peppers and chillis if needed are all possibilities. i sometimes add a sachet of cream coconut and sugar if i wanted it sweeter
Add your sauce, yoghurt or tin toms and gently simmer, stirring occasionally until the meat is just cooked nicely.
Cook your rice...i always add 10 cardamons and ten cloves to my rice as well as a good sprinkling of cumin seeds. remove the cardamons and cloves when cooked.
And that really is as basic as you can get. You could do this in a slow cooker or cheat and just buy a jar of curry sauce.
Remember, whatever you add, do it slowly and keep tasting. You can always add more if needed but you can't take them out once added.
Good luck