It is easy to find Gellért Hill because it is a hill with a hefty woman on its top seen from almost any point of the city. Actually, Gellért Hill is a huge and not very well groomed park, representing the whole variety of local flora (including even fig trees!). Climbing up the hill, you will (a) know your actual physical condition; (b) get a bird's-eye view of the city and (c) sooner or later, one way or another, reach the Citadella.
TIP: You can climb Gellért Hill either from side of Gellért Hotel, or Erzsébet híd (the white bridge). The former ascent is steeper, but shorter. There is also bus 27 from Móricz Zsigmond circle, but you won't get (a) and (b) if you choose this route.