You should have the answer, why you want to do CA? You should have the interest from within, not due to peer-pressure or someone's suggestion or someone acting as inspiration..
I've known CAs who have graduated in Science, so it's not like that CA is the dominion for people with commerce background.
Further, quite a lot of engineers eventually turn to finance field after completion of B.Tech, by way of MBA or other courses or just like that due to their analytical/ mathematical skills.
So if you are going to do that eventually, how is it wrong if you do it now itself? Give this justification to people who raise questions about your sanity.
Just because someone is gifting you elephant (gift is for free) doesn't mean you should take it without considering its maintenance costs, so you gotta do what you want to do.
And I don't think there has been any CA who had known all its life that he/ she would be a successful CA without being successful first. Also it depends upon on how you define a successful CA as...
If someone here tells you, you would be successful and you are roaming in the beaches of Goa through the year, you will surely prove him/ her wrong.
Regarding your preparation, you will certainly have no idea about other 3 subjects unless until you go through them first. It would be easier than building a rocket, that's for sure!
You can also consult some CA or CA student to understand the course first before joining in.
Once you are in, you are in... there's no backing out, no backups.. So think carefully, take opinions of your friends, relative, etc.. If you think they'll confuse you, introspect yourself. And whatever you choose just don't regret, think you had no options in first place and go on..
But, BTW... wow Goa! You must be lucky to be living there! :) And what college you got chance in?