Alloy questions!

I have the urge to upgrade my steelies for a set of alloys. A friend of a friend is going to do me a good deal on some 15" ones. However I know nothing of alloys really (other than what I read during a search on this site).

For a start - what's the default steel size on a Skoda? i.e. will I have to upgrade my tyres whatever the weather when alloys go on?

What size do I use? Does it have to match as existing or can I just whack on whatever takes my fancy?

Who would fit alloys for me? (ie a tyre centre?)

Many thanks!

Jon