Hi check these posts to understand how the forum works
Official Sapienmed FAQ (Updated)
Forum guidelines for beginners
But for your question in particular on how to message someone, you can click on the member’s picture and it will open a small window where you see says “message”
Just know that maybe members dont necessarily love getting direct messages 
We do like helping specially new members but sometimes the questions in private messages are ones that have been answered within the posts in the forum, and is typical that we reply one message and then the messages keep coming one after the other as if one has become their personal guide, and it gets tiring to be honest.
So instead you can ask under any thread of particular cases or fields you need/want information that you havent found anywhere, and that way many members (not just one in private message) could help you and we all interact better that way.
And if you still want to private message someone, then you might want to ask them on a post first if they are ok with that or not.
Dont be shy asking in posts, no question is silly, just browse the forum posts before 
In the links above you can exactly see how and where to find specific threads
Enjoy it!