Dead battery? (again)

[mf4361]

I was trying to start my Yaris this afternoon and wouldn't start :(

At first, it sounded like battery is not have enough power to crank, then after a few tries when i turn the ignition, it gives some grinding noise (scary). Does that simply mean the battery is flat or is the grinding noise indicating something worse happened? The terminals does appear to have oxidation but no luck after cleaning.

Its not starting well since this winter and got worse after I stupidly overdischarge it. Sometimes after leaving it a day or two unused, it had trouble starting. (It would start if I hold the ignition a little longer, but this is the first time it actually didn't start)

Is Walmart's Everstart 26R a good choice? or I'll need a 35N? Or any recommendation?