I read in few papers that introducing some kind of initial disturbance can speed up the vortex shedding for flow past a cylinder in an unsteady flow.I need some ideas on what kind of initial disturbance should i use.Something like an initial vortex or pressure pulse.?

Nothing too complicated. Simply destroy the symmetry of the steady solution to get shedding started. For, example, choose a converged but asymmetric steady solution, say with a 10 degree angle of attack, as your initial condition for the unsteady computation.

Rotate cylinder (first in clockwise direction for some iteration and then in the anticlockwise for some more itereations).. then keep the cylinder stationay and continue the simulations...

Braza etal, JFM.. have give a detailed study on cylinder vortex...