I tend to put whatever I have in! Start with a base of stock - either home made, or stock cube, or you can even buy gels which you dilute, and ready made fresh stock. For a quantity, put all the "chunky" ingredients into the casserole dish and cover them about 2/3 to 3/4 with stock. You can then top this up with other liquids (can include extra water if you have lots of flavoursome ingredients) - the finished liquid level should be roughly level with the top of the veg and meat. I tend to go for slightly under so that they are poking out by a couple of mm, but it depends how much gravy you want.
Then dried herbs, or even fresh if you have any space to grow them - often when you look on the jar it will tell you "Goes well with chicken and fish" or "Complements potatoes and lamb" etc. Supermarkets change their labels a lot so if you can't find any with tips, try looking on jars or packet sauces, in the ingredients, to gain an idea of which herbs and spices work well with which other ingredients. You can also use things like bay leaves and bouquet garni, which you remove after cooking.
Salt and pepper almost always help. You can top up the stock with things like wine, beer, cider, cream/milk, tinned tomatoes, coconut milk, whatever will complement your meal (obviously not all of these at once
) and you can add a dessertspoonful/tablespoonful (for a larger casserole) or so of ingredients like mustard, yoghurt or cranberry sauce etc to add a background/extra depth to the flavour. This is a weird one to judge as it tends to not come through explicitly - I can never taste the mustard in my beef casserole, but it just adds something which is missing without it.
If your main ingredients are fairly plain or limited, try adding some more strongly flavoured things like bacon, garlic, peppers, chillis and onions, again, probably not all together. Lots of different vegetables such as leeks, turnip, swede, parsnips, carrots. I don't like to put potato in things, it soaks up the flavour too much, I tend to serve with mashed or baked potatoes instead.
If you have used mainly very liquid ingredients like stock and wine, you will want to thicken up the sauce a bit. Either stir some plain flour into your saucepan/frying pan when you are browning the meat, and make sure this makes it into the casserole dish or slow cooker, or if you skip this step then mix cornflour with a small amount of water in a jug or mug until it makes a smooth paste and add it in. You need more than you think.
Experiment! If you're nervous, try following recipes, but generally you can play around with quantities of ingredients etc to see what works and what doesn't. I find a slow cooker is really helpful because you can literally just throw everything in, switch on and leave it and even if it's not perfect it's still nice because of the slow cooked nature of it. If a sauce is bland when it comes out, you can always add salt, pepper, mustard etc on plates, and make a note for next time.