The most widely accepted cards in Lithuania are VISA and MasterCard. Some places may also accept Maestro and American Express, you shouldn’t rely on those too much in Lithuania though.
In general, most of the places accept payments by cards, however having some cash is still strongly recommended as you might face a lack of card payment support if you visit any of the following places or similar:
- Some intercity buses
- Buying public transport tickets from the driver
- Street merchants
- Some taxis
As for ATMs, Lithuania offers a very wide network of ATMs of different banks. As a general rule, you should be able to find one at the nearest supermarket. Also, google maps has a short list (far from complete) which might be helpful. Unfortunately, there is no single map of all the ATMs of all the banks in Lithuania.