You don't say much about your system. Labview? C/C++? VB?
What documentation have you tried to read?
For C/C++ you would use DAQmxClearTask, which is in the reference documentation pretty close to DAQmxCreateTask. Other programming languages/environments will use something with a *very* similar name.
John Weeks
WaveMetrics, Inc.
Phone (503) 620-3001
Fax (503) 620-6754
www.wavemetrics.com