Bizarre, I thought I'd already done this, but I can't find it, so here it is.
Soup is the most basic item of food you'll ever make. You can serve it as a lunch, a starter, a main course, whatever. You can use leftovers or you can buy fancy ingredients. None of this matters. Soup is a structural thing.
1. A BASE. That is to say, a blend of flavours that will work with anything. In classic French cookery this is a mirepoix. Cut up some onions (or leeks), some carrots, and some celery, and sauté them slowly/gently (i.e. low heat) in a fat of some sort (butter/oil/any old fat) until they are soft. Other cultures use different bases. Maybe peppers or garlic. Doesn't matter. You need something related to onions and a couple of other things. You can research/study this but you won't go far wrong with onion-carrot-celery, trust me.
2. LIQUID. This can be stock, or milk, or tomato juice, or any combination thereof. In theory it could be water if everything else is very flavourful, but you have to know what you are doing, so if you are a beginner USE STOCK. It can come from a stock cube, so don't panic. Just make sure that when you make it up it tastes fine AS IS.
3. The MAIN ELEMENT. This can be vegetables, or meat, or some of each, singly or in a mix. It could be just potato, or it could be a medley of all sorts. On Boxing Day, or the day after Thanksgiving I make soup from the leftovers. So, having done the above, I throw in turkey, roast potatoes, sprouts, carrots, peas, gravy, and even leftover cranberry sauce, bread, and stuffing. This is the beauty of soup. It can absorb anything. I have even thrown in leftover sausage rolls. You can literally make leftover soup from whatever is in your fridge. Or...in growing season you can get rid of all whatever glut you have.
4. SEASONING. Depending on your stock you may need to add salt. You will definitely benefit from a little pepper. Taste as you go. Herbs? Depends on what is in there. If it's tomato, then basil is good. If it's pork? Add sage. If it's chicken? Thyme or tarragon is good, etc. If you like it you can add spices, including curry spices (potato soup and curry spices are a marriage made in heaven.) Fish? YES FISH. Fish soup is amazing...add parsley. Etc. What do you normally add with this item?
5. THICKENING. If you are seeking a thick soup and your soup is runny, then add potato or flour. If you want to make it a main course soup you can add rice, pasta or something trendy like quinoa. These can be blended as can any/all of the above. If you add flour, stir it into a little cold liquid first or you may get lumps. Then add this mix while stirring.
That's it. That's soup. Any soup you ever enjoyed was made this way. Sure, you can use cream/yoghurt/sour cream at the end, or wine or vinegar to change the flavour, but this is basically it. Leftovers? They go in as #3.