First of all, there are 3 interlinked stations, so be sure which you are transferring between: “Stockholm Central”/“Centralstation” for main-line trains (including the Arlanda Express); “Stockholm City” for commuter trains (Pendeltåg); and “T-Centralen” for the metro (T-bana). “Stockholm C” usually means Stockholm Central, but occasionally gets (mis)used for Stockholm City.
If you’re just transferring between main-line trains, jcaron’s answer gives a good map of the platforms of Stockohlm Central. The main station concourse building is on the west side of Vasagatan and south of Klarabergsviadukten; all platforms are clearly signed from there. Platforms 1–10 are reached from the Vasagatan street level hall, while platforms 11–19 are reached from a lower level concourse below it.
If your change also involves the Pendeltåg or the T-bana, then the 3-dimensional diagram below is your friend. The underground platforms are a bit of a warren, but signage is pretty decent. In particular, signs towards the metro and Pendeltåg are very clear, so just follow those if that’s the way you’re going. Signs towards the main-line Stockholm Centralstation from the T-bana/Pendeltåg are reasonably clear, and mostly will bring you into the lower level of the main station concourse (via the passage at the top-right of the diagram); all main-line platforms are reasonably signed from there. A few signage routes may lead you to Stockholm C by some other entrance — e.g. the northern exit from the Pendeltåg or the metro blue line (bottom-right of the diagram, “Biljetthall Vasagatan”) has a passage bringing you up near platforms 1–3 (where the Arlanda Express goes from).
If you want any more details/advice, please tell us which trains/lines you’re transferring between, and I’m happy to elaborate further.