Do we need to pay taxes or anything else in mturk?

It depends on the laws of your country, as well as what Amazon's policies are. I know for certain (most) programs, if you made more than $600 in the entire year, you have to claim the income and pay taxes on it. This is how it is in Minnesota (of the US) though, I"m sure it's different state to state and definitely country to country. I know that I haven't come close to earning over $600 on a single program, so this doesn't affect me at all.

Amazon require everyone to either declare that they are an American citizen (and therefore liable to pay taxes to the US) or not an American citizen (in which case Amazon do not need to pass on information to the IRS about those earnings). Amazon do not deduct taxes. They are simply required by law to pass on information about Americans only who earn more than $600 in a year to the IRS. That is why they need a declaration from you that you are NOT an American citizen/resident. Most countries require that ALL earnings should be declared for tax purposes. What you do is up to you but you would be well advised at least to keep a detailed record of your earnings and expenditure just in case your tax authorities decide to investigate your earnings.