Much of cap selection is by 'rule of thumb' and learned by experience. This is mainly for the by-pass caps that are placed across the plus and minus pins of ICs.
Caps are used for difference reasons in circuits so one needs to understand why a cap is in that part of the circuit. There are equations when a cap is combined with other passive components such as resistors and inductors. Look up RC, LC and RCL in Wiki for equations.
Circuit simulation tools (programs) are also used to determine cap values. LTSpice (free from Linear Tech) works well and is nice for visualizing the AC response of a signal with an RC, LC or RLC components.
Good question but there isn't a quick easy answer.