Don't be that person who pays for every single disparate train ride. Don't be that person who pays full price for museum tickets. Just, in general, don't be that person who didn't opt for a travel card or travel-card-like solution. Many cities have very tourist-friendly bundles specifically designed for tourists looking to travel and see a lot in a short amount of time. You should definitely do yourself a favor and check those out.
And if you're having trouble finding those deals, which may just happen given that you're going to travel to a foreign country you know nothing about, you can always scour the web for advice.