Good evening Clara,
I always recommend that first time visitors take a guided tour, as a way of getting a sense of the layout and history of the city, as well as good tips from the guide for where to go and what to see if your own time.
As a general rule, do the Charles Bridge in the early morning and the castle at night - that way you'll avoid the crowds on both.
Go see a show in one of the opera houses of the National Theatre: The Bartered Bride, Nabucco and Oliver Twist are all playing on the 26th. Bring your glad rags and go to the opera or ballet.
Soak up the atmosphere along the riverfront on Wednesday at the Naplavka, as the young Czechs gather to drink beer and play music and relax on the quayside. Have a look at the farmers' market on Namesti Jiriho z Podebrad on Thursday and check out the DOX gallery in Holesovice on Friday.
I hope that this helps.
Welcome to Prague.
Marcus