My food doesn't have too much taste / taste allways the same? [closed]

I don't know where you are living or the quality of the food you can get your hands on, but these are (obvious) generalities that should be true:

• Ingredient quality plays a lot, for example most tomatoes sold in supermarket are just reddish water bags without any taste. You can remedy this either by looking for better quality ones or by adding tomato paste or some kind of tomato concentration. Ground meat, depending on the fat concentration, freshness, etc., can range from fast food leather to deliciousness.
• Preparation is something really complicated. Just for the onions there are more than a couple of options for the preparation, did you make your onion sweat? Did you sauté them? When you did add the tomatoes, did you let them reduce and form a sauce? Did add garlic at the very end - the garlic flavor is destroyed really quickly by heat - or if not, did you put enough for it to survive the whole cooking process?
• It might just be that you are preparing things in the same "style", hence the impression things are more bland than they might be. Try truly different ones. It might help.
• To get really tasty meals, salting is preponderant. However most people underestimate the quantity. By a factor ten or so. Salting at the end of the cooking in the plate is really not ideal too in most meals. You should taste and adjust salt at every step of the preparation. A rule of thumb I heard in kitchen is 10g of salt per kilogram (Measure it in a scale; that's a lot more than what you might think).
• Or it is the reverse and it might just be cultural apparatus, used to eat meals heavy on sugar or salt you need time to adapt to the normal taste of things.

As you said the question is really broad, hence the answers are too. Try to understand where the issue comes from by following recipes and then experiment with each factor until you get it right.

To build on what Sharnt said (and yes, this is very broad)--

Grocery store tomatoes are really awful. They might be okay to bulk up already flavorful things (eg, on a taco), but that's about it. Even the 'vine ripened' (ie, showing a little color when picked) ones that you pay more for are really a pale imitation to ones that you grow yourself or get from a farmer's market when they're picked fully ripe. Unless you specifically need raw tomatoes, you're often better going with canned. If they're too wet, you can always drain off the extra liquid. (but reserve it, in case you need to add some moisture back in).

Preparation also has a huge influence on things, especially on aliums (garlic and onions). Cooking them low & slow will bring out their sweetness and destroy the flavors that you get when they're raw ... a head of garlic allowed to cook slowly in the dish might give less 'garlic' flavor than a clove smashed and added at the end. For more info, see Is there any difference between chopped and crushed garlic in cooking?, How does the way that I cut my garlic affect the taste of my food? and (Why) do onions taste sweeter when cooked at lower temperature?.

Cooking over higher heat will also destroy some of those qualities, but not has much. It can also result in browning, which if stopped before burning can create more complex flavors. See Why sweat but not brown?.

Order of cooking can also play a factor -- brown the meat first, then add the onions (and possibly garlic) and cook it for a little bit, then add the tomatoes. How far you cook the onions before adding the tomatoes affects how firm the onions are (the acid helps keep them firm), and how much you develop the sugars in the onions ... or even caramelize the sugars in the onions if you keep the heat up and brown them.

Also note that there are many varieties of onions available -- yellow, red, white -- and some are 'sweet' (ie, less onion flavor) they tend to be paler yellow and less spherical (ie, a bit squashed). See What are the differences between different types of onions, and when do you use them? and What's the difference between green, white, and red onions?.

And similar to sweet onions, there's also elephant garlic, which has less garlic flavor, which is significantly larger bulbs (and not technically garlic).

It's also worth noting that spices can go stale on you. If the nutmeg is already ground, try a pinch of it, and see if it tastes like nutmeg (or anything, really). You can try adding them early to wake up the flavor during the higher heat part of the cooking. If you were using dried herbs, you can pour it into the palm of your hand and then crush them with your other hand to try to get more flavor out of it.