Is learning Telugu script need any extra effort if I already know reading Kannada script? since both languages are quite similar for reading.
Yes, learning Telugu script is a little bit of effort, but they are really similar. I would think you can learn it in less than a day.
Here is the basic Kannada script, which is sounds like you already know:
And here is the basic Telugu script, which you want to learn:
They are both Dravidian languages, and the similarities are everywhere in these scripts. The main differences appear to be: u, r gha, na, ta, śa. But even those letters have some similarities. Overall, it seems more like a matter of "getting used to" the Telugu script, as opposed to learning it from scratch, if you are already well versed in the Kannada script.