如何计算tabulated value required for a 95% confidence interval

如何计算tabulated value required for a 95% confidence interval
A random sample of size 30 is obtained from a normally distributed population.The mean of this sample is 38.1 and the standard deviation of this sample is 8.5.
Since sigma,the population standard deviation is unknown,we shall have to calculate a confidence interval using a critical value of the t distribution with n-1 degrees of freedom.
1.What is the estimated standard error of the mean for samples of this size?(2 dp)
2.What are the degrees of freedom for this (integer)
3.What is the tabulated value required for a 95% confidence interval?
(3 dp)
Based on this sample,the 95% confidence interval for m is given by:
(lower limit,upper limit) = (_._ ,_._)
4.lower limit (2 dp)
5.upper limit
应该是很基础的题就是第3问不太明白怎么算,能简单介绍下基本定义么