TestComplete supports 5 scripting languages for creating tests: VBScript, JScript, DelphiScript, C++Script and C#Script. VBScript and JScript are standard Microsoft scripting languages that are used on web pages and for running Windows Script Host scripts. DelphiScript is a similar language provided by SmartBear that has the Delphi/Pascal syntax. As for C++Script and C#Script languages, they are just modifications of the JScript language and are needed only if you are working with C++ or C# Connected Applications.
So, if you are familiar with Java, the best choice is JScript. You can find the language reference in the MSDN Library.