Not bad. :) There is no one who can't understand yours but these are more natural; 오늘은, 나 학교에 왔어. 오늘은 학교에 왔어. 오늘, 나는 (or 난) 학교에 왔어. 오늘, 나 학교에 왔어.
* '은', '는' are topic marker. A topic marker is a grammatical particle used to mark the topic of a sentence, so it sounds a little awkward when topic marker is used more than one in a sentence. 오늘은 <= you are talking about 오늘(today) ; 오늘은 blah blah~ 나는 <= you are talking about '나(me)' ; 나는 blah blah~