I have calculated the one sided t-test statistic and am getting a negative value as the sample mean is less than the mean under H_0. there are no negative t values in the table - do i take the absolute value of this?
Take the negative of the critical value in the table and reject if your t-value is less than this.
I'm presuming you're doing a one-sided test on (<). In this instance you could take the absolute value and reject when t is greater than, but you can probably see how always taking the absolute value can go wrong for a one-sided test?