Water quality is affected by two main factors: firstly, contaminants (organic and inorganic) which it contains when drawn from the tap, and secondly those which enter it while it is in the aquarium. Both inorganic and organic contaminants are involved.
Inorganic contaminants
Tap water may contain small quantities of various metallic salts, either naturally or as the [...]

Plain, cold tap water is acceptable, pretreated to remove contaminants if necessary. For your first aquarium, you will probably be buying fish locally, and they will almost certainly have been kept in local tap water like yours (but check!). It is thus usually unnecessary and undesirable to modify your water chemistry at this stage, even [...]