From Proto-Italic *telnos, perhaps from Proto-Indo-European *telnos ~ *telnes-, from *telh₂- (“flat ground”), but reshaped after rūs. Otherwise, could derive from Etruscan 𐌕𐌖𐌋𐌀𐌓 (tular, “earth”).
Practice speaking and memorizing "tellus" and many other words and sentences in Latin.