It's called the 3-2-1 rule, and it's the easiest way to remember the regulation. To recap, if the weather at your destination isn't at least 3 SM of visibility and 2000' AGL ceilings from 1 hour before to 1 hour after your ETA, you need to file an alternate.
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Standard takeoff minima are one statute mile for aircraft having two engines or less and one-half statute mile for aircraft having more than two engines. Specified ceiling and visibility minima will allow visual avoidance of obstacles during the initial climb at the standard climb gradient.
If a usable precision approach is available: 600' ceilings AND 2 SM of visibility. If only a non-precision approach is useable: 800' AND 2 SM of visibility. If no instrument approaches are available: Descent from the MEA and landing must be conducted under basic VFR.