Firstly, please don't try to force anyone to do something they might not like/be into. If you want your boyfriend or wife or partner to be into dirty talking and you don't think they are--talk to them about it. Sit them down and say "hey, babe, it'd really turn me on if you talked dirty to me sometimes. Would you be comfortable doing that with/for me sometime?" And if they say no, respect that.
I have trouble with this too (my partner just isn't very vocal, and I'm a bit shy at times). And I agree with kadelette's idea- to ask questions that will lead to the answers you want.
A good thing to keep in mind with anything you want in the bedroom is to talk about it during a time not related to sex. This gives your partner the freedom to discuss any doubts, and not feel obligated or uncomfortable.
Also, just try to relax and take a deep breath before you talk to him. I'm sure he will appreciate your honesty about it (particularly if he knows you are shy and that it is hard for you to talk about) and want to try new things with you to make you happy.
How to make my boyfriend talk to me dirty ?! I love that but i'm so shy to tell him talking to me that way ... hope to help me
The best way to really get him to do it is just tell him. The first time exchange dirty talk with each other and reward him for doing so. The positive reinforcement works wonders on those who are more shy...