ullasckm, this is not a stupid question at all... in fact, it's a very common question, and almost everyone who registers a domain considers this question in some capacity.
If you can afford it, I would register .com, .org, and .net
And if you can also afford it... if you domain contains more than one word (e.g. mydomain.com), I would also consider registering one with the dashes (e.g. my-domain.com).
All this, as I am sure you're wondering, would add up to a lot of cash, so don't spend more than you budgeted, you can spread buying them over a month or two
Remember, this is what I would do, and may not be the best thing...
You can always just register the .com and not worry about it
Sherif