installing custom rom help noob

hi guys, since samsung s2 i dont use samsung mobile phones so its a long time

i bought a galaxy s6 and the original room is full of aplications i dont use, the ram avaible is a joke and the battery life isnt very good

i want to install a custom rom

im with the last ofical 5.1.1., if i install a custom rom i will loose right to waranty? there isnt any way of revert the root and count?

thanks