i can devot around 1 hr per day and 2-4 hrs on weekends for coding. but i finding it difficult. what should i do . specifically corner test cases .

so

how do i start preparing for Zonal computing olympiad (Zco)?

If you say you only have problems with the corner cases but you find a way around the problem as a whole, then the best thing you can do now is practice. Because from a point of view, you cannot theoretically understand not missing corner cases. I would say you solve as many problems as possible from the past papers of ZCO and INOI, you will find them in the “Compete” section of codechef. Also after you are done, you can proceed to solving more problems in the practice section and also you have codeforces. 1 hour per day is what almost everyone gets, even I get an hour a day, even on weekends.

I hope I could answer you, if you have any more questions, you can ask here, I will try my best to answer.