How much the unit will do depends on the mineral content, and more importantly, the quality of the unit. Cheap one stage units will reduce the mineral content by 70-80%. Quality multistage units will reduced the mineral content to virtually zero.
An RO unit is normally plumbed in. It requires high pressure water in to create the osmotic effect across it's membrane. There are 2 outlets, the RO water outlet and the waste water outlet. The waste water will be greater volume then the RO outlet. If you are on a metered supply, you will probably want to do something with the waste water rather then dumping it down a drain.
RO water from a good unit is unbuffered and not suitable for use in a tank without adding stabilising salts.
Remember if you run your tank at very different water chemistry from your fish suppliers, you will need buffer tanks to aclimate your new fish from the tapwater your lfs probably uses, to the RO regime in your display tanks - the need for this should not be under-estimated, tap water fish introduced into a low mineral RO setup can suffer fatal osmotic shock.
Adjusting water chemistry requires a reasonable knowledge of the chemistry involved if it is going to be successful. Unless you have a specific need for low mineral water, (breeding for example), I would not normally recommend people do it.
