Generally speaking, you want signal-modifying effects in front of the preamp, and time-based effects after it.
If everything must go in front of the preamp (no effects loop available), then time-based effects go last.
If you have a loop, they generally come two ways: series or parallel. Sometimes, you can switch them to be one or the other. But, the same ruleset applies.
Signal-modifying effects are things like power boosters, distortion, fuzz, overdrive, filters (equalizers and wah pedals), compressors, etc.
Time-based effects are things like reverb, delay, flange/chorus, etc.