"I will sing a song today."
How did you make 歌いる? Replace the ます of 歌います with る? Duo is too stuck on using Japanese script to tell us, but the root of the verb is "utaw-." If you attach -mas- to a root ending with a consonant, you throw in an -i-. For the simple non-past of a consonant final verb you add -u, so 歌を歌う (utaw-u).