The Old Town, Josefov, the river and Charles Bridge are all within walking distance. The streets are cobbled, so wear shoes you already know you can trust, and steer by the river and the squares rather than trying to walk in a straight line.
Trams are how Prague gets about, and they come often. Let people off before you get on. It is often nicer to ride a stop or two past where you are going and walk back along a quieter side street.
The metro is for the longer hops, after which you are on foot again. Some of the stations are very deep, so allow a few extra minutes.
Tickets are valid for a set time rather than a distance. Buy an official one and keep it until your journey is over, because even a single stop needs a ticket. A car makes sense late at night or when you have luggage, and if you take a taxi, agree the fare before it pulls away.