Because "στάση" is considered a female now, so η is the article to go with. Το is for neuter, so "το στάση" would be grammatically incorrect. I hope I helped you ^.^
On a standard QWERTY keyboard, when switched to Greek, you first hit the key next to L and then, straight away, the key of the vowel you want to add the accent on: ; + α = ά. You won't see anything before you hit the vowel key though.
Whenever you learn a new noun find it's gender as well as learning the spelling of the noun by putting the word (the) before stop etc into Google translate. Just to be sure I put in The stop, one stop and a stop. η στάση, μια στάση, μια στάση. When you are reading Greek sometimes they omit the a or an and just put in the noun so as not to confuse it with (one). και γίνει σαν παιδί, and become like a child. The ένα is left out so as not to confuse it with one child. ένα παιδί.
DIMITRA956826 I thought "stop" was a verb of action rather than a noun. But I guess if it's "the stop" then it must be a noun of subject. It's feminine, so "η" not "το".