Marc is quite right Note: Although some will argue that 'normal' is a bogus idea, it's still the starting point for much general photography. Optically 'normal' has long been defined as the diagonal of the frame (film or sensor). For a 6x6cm MF cam, that's 80mm. For 135/FF cams, that's ~43mm. For 135/HF and APS-C cams, that's ~30mm.
[Acronyms: 135/FF is full-frame, like most 35mm cams; 135/HF is half-frame, about the same size as the APS-C sensors in our dSLRs; MF is medium-format, for cams with ~60mm wide frames.]
Ah, but so many 135/FF cams came with 50mm or 55mm or 58mm glass as the 'normal' kit lens. It turns out that such Fifty's are straightforward to design, and many lens designs in the 40-48mm range interfere with the swinging mirror of an SLR. Thus we have the short-tele Fifty's as "long normal" (and they're quite good in that role). Another factor: 645 and 6x6 and 6x9 MF cams have much bigger frames than our film and digital SLRs, so there's more leeway for cropping. Think of 50mm use in a 135/FF cam as "pre-cropping".
But if ya wanna get normal with an APS-C dSLR, 28mm is the closest, cheapest. Some will call that a "short normal", and a 35mm (about equivalent to 50mm on 135/FF) is "long normal". Whatever. Some think that 35mm on 135/FF or its rough equivalent, 24mm on APS-C, are wide; others, that they're slightly wide; others, that they're 'normal' for their use. Whatever. Decent glass is widely available and often quite-to-moderately cheap in the 24-28-35-37mm range. For a nice sharp rough equivalent of a classic Tessar 55, try the M42 manual Mir-1 37/2.8. You'll be glad you did.