Lithuanian currency is Lithuanian Litas (LTL). It is currently peged to Euro where 1EUR = 3.45LTL.
You can change money at any bank. Take euro or usd as they have the best rates.
Traveler cheques are usualy not accepted (except for some top-end hotels and restaurants), but there is no problem to cash them at any bank or exchange office. There is 24hours a day opened exchange office in between bus and train station in Vilnius. Good rates.
Visa and Mastercard are the most popular and are accepted almost everywhere in major cities. ATMs are available everywhere throughout the country, so you should never run out of cash :)
10-15% are expected in most of the places where you eat or drink.
Only in markets. You can also kindly ask for a "discount" at some hotels
It depends on a place. The most expencive is Vilnius, Kaunas and Klaipeda. Everywhere else it can be up to 50% cheaper. Some sample fares (in Vilnius):
Pint of beer: 1-2EUR
Bottle of beer: 0.50EUR-1EUR
Coke 0.5l: 0.5EUR
To be continued..