I want to paint a shelf in White , and a mask in it like image attatched. Should i use oil based or acrilic?
Ignoring the masking in of the mask design for now and answering original question.

The choice of paint is up to you. As fixer1234 noted you are better off using oil based enamel paint as it gives a harder more permanent finish but latex will do if you want to avoid solvents and messy cleanup. 

You would want to sand and prime the board first. Match the primer to the paint you plan to use. Sand after priming, maybe use two coats if the wood is extra thirsty or the old finish still shows through. Same goes for your finish coats. 

To add a design, make a stencil or hand paint it after applying the base coat. Be sure if you are using a different color that it is the same type of paint (latex or oil enamel).  Then apply a clear coat to protect it, also of the same type of paint. 

I am not going to go into paint brands or how to prep and clean brushes etc... that you can find out for yourself with a little basic research.