The most common salts to add are calcium chloride or calcium sulphate (gypsum). If your water is low in magnesium, you can add epsom salts. Most people tend to use the first two.
I don't think you need worry too much about ruining your beer. Whilst salts do have a noticeable impact, we tend to use a few grams of them in ~25l of beer and the impact is moderate to low (in the grand scheme of other things).
If you want to play/test/experiment, you can also add the salts to the finished beer. If your stout is low on mouthfeel palette, then measure a small quantity of calcium chloride and add it to your glass and get it to dissolve. Then taste the beer and see the difference. Even better if you do a side-by-side comparison with an unsalted one, and get someone else to serve you so you can test blind and see whether it actually makes that much difference to you. If you measure out the weight of a tablespoon (15ml) of the salt on accurate scales, that will give you an idea of what a tablespoon weighs. Depending on how small your measuring spoons go you can use this to approximate how much to put in a glass. eg, I have a 1/8th tsp measure. 1/8th tsp is 1/24th of a tablespoon, which works out at pretty good downscaling from a full batch. Eg, 1 full batch is (roughly) 24 litres, so if I were to put 1 tablespoon of "something" (roughly 10g of yeast from a recent test I did, but probably many g of salts) then the equivalent 1/8th tsp would be the same proportion for 1 litre for the test, so half of my 1/8th tsp per 500ml bottle (I don't have a 1/16th tsp measure)