Calculation of forces

You must refer a Theory of Machines textbook for learning how to analyse forces acting in a slider-crank mechanism. Procedure too lengthy and will vary according to the information given.

Kelin's construction is one of the methods used

This topic is much too involved to get into here - especially with as little information as you have provided. Here are some examples:

http://gemini.tntech.edu/~slc3675/me361/lecture/foranal.html

Assuming you are a student, you might try doing your own homework...