I'm just starting to learn Russian and feel overwhelmed by all the different forms of words. When I learn a new word should I try to memorize the different cases for nouns or just the nominative and trust that I'll be able to figure out the others as I improve as long as they aren't irregular? Should I learn the masculine, feminine, and neuter forms of adjectives or just one? Or is it better to learn complete sentences?

Thanks for your help.