There are many online sources that provide information about the Lithuanian alphabet. An example is the following:
http://en.wikipedia.org/wiki/Lithuanian_alphabet
In general, the Lithuanian alphabet has the following differences from the English alphabet:
If you find the letter "W" or "w" in your ancestor's surname, that letter was likely a "V" or "v" originally.
The Lithuanian letter-combination "ch" is not pronounced like the "ch" in English "church"; it is pronounced like the "ch" in Scottish "loch".
The Lithuanian letter "č" is pronounced like the "ch" in English "church".
The Lithuanian letter "š" is pronounced like the "sh" in English "sheep".
The Lithuanian letter "ž" is pronounced like the "Zh" in "Dr. Zhivago" or the "s" in English "pleasure" or "treasure".
The Lithuanian letter "c" is pronounced like the "ts" in English "pants" or the "c" in English "dance".