Scenario 1: Calculate Mean Difference and Standard Error Between Two Groups, Suitable for Direct Use of Effect Sizes in Meta-Analysis[1]
Group 1: Sample Size Mean Standard deviation
Group2: Sample Size Mean Standard deviation
   Confidence level

Result: Mean difference Standard error
Confidence interval    Lower limit    Upper limit
Scenario 2: Estimate Standard Error and Confidence Interval
Based on Mean Difference and its P-value [2]
Mean difference
 P value
                    Confidemce level

Result: Mean difference Standard error
Confidemce interval Lower limit    Upper limit

Reference:

[1] Rosner, Bernard (Bernard A.). Fundamentals of Biostatistics. Boston :Brooks/Cole, Cengage Learning, 2011.
[2] Altman, D. G. and Bland, J. M. (2011). How to obtain the confidence interval from a P value. BMJ, 343:d2090.