If your phone doesn't support Tamil natively (e.g., you can't switch the menus to use Tamil), then it won't support Tamil in MIDlet UI components either. In this case, you could still display Tamil characters in a MIDlet by including the font as a PNG image file in the MIDlet's JAR file and drawing the characters on a MIDP 'Canvas' (use a clip window to make only the needed character get drawn). However, entering text in Tamil would probably be very clumsy.
If the phone does support Tamil natively, then your MIDlet should just automatically work in Tamil too.
Java language characters and Strings all use Unicode, so a MIDlet should work in Tamil without modification: just use Tamil characters for all the messages you display, and e.g. TextFields will use Tamil for input. If your messages are in your Java source code and that source code is in ASCII or ISO-8859-1, you'll have to represent the Tamil characters using Java's Unicode escape sequences, e.g.: '\u01C9'.