Depends what you're brewing Bada.
Ask the same question about yeast, grain or hops and the reply would be similar.
Basically:
-You need drinkable water that tastes and smells OK.
-You need to know what ions your water contains - particularly calcium, chloride, sulphate, magnesium, zinc, sodium. You need to know the temporary and permanent water hardness and whether it's sanitised with chlorine or chloramine. If it's chloramine, you may need to filter with RO or treat with bisulphites/campden. If it's chlorine, boiling in preparation, allowing to sit overnight or even just bringing to strike temp should be adequate. Charcoal filter works too. Worry about means other than normal heat to strike IF you start tasting band-aids in your beer.
-You need to try and drop out anything excessive, with particular consideration being given to bicarbonates, magnesium and sodium. Palmer talks about ranges of ions and his ranges generally link up with brewing science texts I've read.
-Main things you need to make decent beer after this are - proper mash pH, sufficient calcium and a chloride:sulphate ratio that fits the style of beer you want to make.
Proper mash pH should be between 5.0 and 5.5 when measured at mash temp and between 5.3 and 5.8 when measured at room temperature.
Dark grains raise mash pH, paler grain mash will therefore have a lower pH.
Ionic calcium salts will drop pH with the exception of calcium carbonate as the carbonate will fight the calcium. You need to measure the mash pH at least once with a mid coloured beer to get an idea (I'd say most times you brew a recipe first time would be better if not always). Acid (phosphoric, lactic and citric) are all commonly used to drop pH when necessary. Acidulated malt and some technical process can also be used.
*Sulphate to push hops (in the form of calcium sulphate or gypsum)
*Chloride to push malt (in the form of calcium chloride)
*Zinc to help yeast and head retention (in the form of zinc sulphate, zinc chloride or decent yeast nutrient)
*Magnesium and phospate levels should be provided by the mash so while those ions are important in the right levels, you shouldn't need to add any.
*Play with carbonates in darker beers if you want - my experience hasn't been good but the DR above reports success whn judiciously used. Your experience only can tell but judicious use is key either way.
As you can see - a simplified summary (and I'm not sure if I've covered everything) covers a lot of ground BUT once you learn the principles, water adjustment can be as easy as:
2g of calcium sulphate and 2 g calcium chloride added to a mid coloured 20 L final volume batch that has a good malt and hop balance. Switch to all calcium chloride for malty, all calcium sulphate for hoppy. Add the same amount to the kettle to push extra flavour components
Paler beers: touch of acid, darker beers either add roast grains later or look at calcium carbonate (if pH is low - test first).
It also can be made as simple as trying a bit out - as mentioned above, it's like cooking. You cook with salt and a cooking nerd or chef might understand some of the chemical processes that occur as a result but you can easily chuck a bit in and learn 'too much' or not enough' fairly quickly if you are smart.
Wessmith: good point. Knowing your water, mash and recipe is a really good way to start. It's not something to really try and work out on the fly, at least not in regards to pH. Possibly in terms of kettle/flavour additions you can make more snap decisions but presumably (and this I'm not sure of) wort pH is important and may be changed by the wrong salt additions??