How to say NO to someone???

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i am really upset over this issue.everwhere in my life..in studies nd else where people used to give me their tasks to do.In our group projects my friends and fellows used to request me please you do it all yourself...will you please do it for me...i m fed up of all this.I dun know how to refuse them taht i will not do..Most of the times i happily do so and feel pleasure in it but some times i have my own pending work to do and nobody bothers.i dun know how to handle such situation...