how to add zeros

i have this type of plot .on x axis time in seconds. the required wave lie in time between 06:00 -07:20 sec. so i want to cut it by zero padding....so my question is how to find sample in this time range...

[Walter Roberson]

first_non_silent = find(abs(YourSignal) > tolerance, 1, 'first');

Due to background noise, you would not typically look for the first non-zero location, and would instead look for the first location whose value exceeds the noise level.