There's a lot of sound advice in the preceding posts, but if you are still a bit unsure then keep it simple, and deal with the important things first:
1/ If you do nothing else then at least put 1/2 a Camden tablet into the whole volume of your brewing water. This will neutralise the chlorine and chloramines. I leave mine overnight, but as long as it has fully dissolved you can use the water immediately.
2/ Download a water analysis report. They are not easy to interpret, but if its anything like the United Utilities reports in the NW, they will tell you if the water is hard or soft. If it's soft, i.e. low Ca and low Mg then it's relatively easy (using one of the many available calculators) to work out what salts you may need to add to achieve the water profile for a particular style of beer. If its hard, then it can get a bit more complicated. You can preboil the water to precipitate out the salts contributing to the hardness, but this is energy intensive, and more expensive now. So, I would suggest that you just leave it alone.
The correct salt/ion balance does affect the flavour profile of the beer, but not by so much that a failure to do it right, or even anything at all, will ruin the beer. You will still be able to make a very drinkable brew. Probably the only exception to this is a pilsner style lager because the flavour profile is so subtle that every fault shows up.
Good luck