Entering Lithuania is no different to arriving in any other EU country. You'll barely notice you've crossed a border if you arrive from another country in the Schengen Area. Arriving from EU countries outside the Schengen zone will involve little more than a passport check as visas aren't required for many nationalities, including the USA, Canada and Australia. If arriving from outside the EU, you may be asked to produce proof of travel insurance.
EU customs rules apply to Lithuania. There's some information on the Lithuanian Customs Department (https://vls.lrmuitine.lt) website and a more comprehensive guide at www.iatatravelcentre.com/lt-lithuania-customs-currency-airport-tax-regulations-details.htm.
Not required for citizens of the EU, USA, Canada, Japan, New Zealand and Australia for visits up to 90 days.