This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Is your server expecting text or a JavaString object? Using an Object Output Stream will serialize a Java String Object.
You should probably take a few moments and run through the Java Socket Tutorial and the Java IO Tutorial. A solid base in those two features is essential for working with sockets.