The most obvious way is to have the first servlet compose a URL and make a connection using the standard Java library methods as if it was a browser. See java.net.HttpURLConnection and related classes.
Your question is missing all the details which might permit a better answer. For instance, are both servlets housed on the same container?
Bill already mentioned one of the details that missing in your question. And that's just a start. You haven't told us anything about why you want to call a servlet from another servlet? One liner questions usually don't get any useful answers because it forces the readers to do the guess work.