problem managing a subordinate

hi to all,
i m a project leader in my current organization which is a small software firm.
I control a team of 4 people.8 months back i was a member of the team and den i was promoted to lead position.i m the youngest member in the team.my problem is that many times they refuse to listen what i m saying or instructing to dem.when i say this work has to be done within today some of them try to delay it by giving various logics.Sometimes they argue with me ,too.u can say that i hv 50 % control on them but i want at least 90 % of control.would be glad if i get any suggestions from u.thnx