First of all there are no stupid questions, only questions to which you may not know the answer
While dan's answer is almost correct it may not be totally correct for you due to us not knowing which 100mm lens you are considering.
The size of the filter you need for your new lens will need to be exactly right and generally saying that 58mm is a common size will not cut it if your 100mm lens needs a 77mm filter
You have not filled in your camera gear list nor written which make/model of 100mm lens you are looking at, without that information the replies that you will receive will be at best a guess and more than likely could be completely wrong. If you wish to supply that information we can much more accurately answer your query
Otherwise if you have not bought the lens yet, all you need to do is buy the filter at the same time and make sure that the supplier knows that you want the filter for the new lens
HTH