Traditional statisticians would advise against calculating the mean on Likert-scale data, which is ordinal data. Numbers represent choices in Likert-scale data, such as strongly agree = 1 and moderately agree = 2. Ordinal data does not have equal distances between the numbers and therefore violates one of the assumptions necessary to use the mean as a measure of central tendency. The median or the mode are preferable. If you are looking for general trends and there is a large sample, however, such as in market research, calculating the mean is acceptable.
Assign a numeric value to each response. Response choices and number assignments could be strongly agree = 2, moderately agree = 1, neither agree nor disagree = 0, moderately disagree = (-1) and strongly disagree = (-2). The numbers could be coded in any way, the numbers here are just used as a symbol for the response --- however, coding with a middle value of zero makes results easier to interpret through graphs.
Add the total responses to each question. If six people responded yielding answers of (-2), 1, 1, 0, 0, and (-1), the total would be (-1).
Divide by the total number of answers: (-1)/6 = (-0.167).
Interpret results. As none of the response choices corresponds with (-0.167), there are two ways to interpret. You could say that most people responded "Neither Disagree or Agree", which is the rounding method, or you could give a range of value, such as most people responded either neutrally or with moderate disagreement.
Be careful when calculating the mean with ordinal data, such as Likert scales. If precision is needed the median or mode are better measures of central tendency.