A bigger problem is that i couldn't find, or either, couldn't be sure of the discounts offered in Poland... I have seen various discounts, but they all differ and don't know which i can trust.
Well, here we have two separate questions to answer: a) question regarding discounts in case of international ticket Poland-Lithuania and b) question regarding purely domestic journeys within Poland. In case of international ticket neither ISIC, nor your age will give you any discount, because ticket (like Warsaw-Kaunas/Vilnius) will be already discounted (50% off Polish part and 55% on Lithuanian) from the standard tariff, it is offer which is used to be called "Vilnius/Warszawa Specjal". To put it in numbers, if we take 2nd class ticket Warsaw-Kaunas, Polish part will be 14 euro, while Lithuanian - 7,56 euro, thus making it in total 21,56 euro for international ticket Warsaw-Kaunas bought in Poland (or Lithuania). With question "b" --> if we speak about discounts for domestic journeys in Poland, like Warsaw-Trakiszki (last Polish station before Lithuania), then there is a different story. Official student discount (51%) here is given only for students studying in Polish universities or Polish citizens studying abroad, so from the first look you aren't eligible, although our Polish colleague filu brought examples when this rule wasn't really obeyed thoroughly (
http://europetrainsguide.com/forum/index.php/topic,331.msg1253.html#msg1253), thus even non-polish students had a chance to benefit from rather essential 51% discount. Obviously it's a bit risky to ask for the ticket with discount, but officially not be eligible for such, however it might still work out fine.
I am a 20 year-old student travelling back home.Does this entitle me to something?
Besides 51% student discount (as explained above), for journeys within Poland there is also so called youth discount (37%), but again, to our knowledge, it also concerns only Polish citizens or those who attend school in Poland. As result once again, by age limit you ok, but trouble is with "polishness" factor
In lithuania students have 50 percent off in travelling
It's a bit tricky discount, because officially (and note about it can be found in almost any LG ticket counter) such student discount is given only to EU citizens studying in EU universities
...so if there is no discounts in Warsaw i can buy another ticket at Sestokai?
Thing here is that full fare domestic PKP ticket Warsaw-Trakiszki costs 62 PLN (app.15 euro), so if you take into consideration potential 51% discount, then definitely it makes sense to buy three different tickets - PKP domestic Warsaw-Trakiszki for 30 PLN, then short cross-border Trakiszki-Sestokai (3,1 euro) and finally LG domestic Sestokai-Kaunas (or further) with student discount. First two in Warsaw (or on board of the train), while third one already in Sestokai during train change. But if to disregard potential discount for Warsaw-Trakiszki ticket, then obviously "ticket break" via border won't be as money saving