I am doing a research proposal and I am stuck on estimating mean differences. To measure the effectiveness of the role-p

[answerhappygod]

I am doing a research proposal and I am stuck on estimating mean
differences.
To measure the effectiveness of the role-play
intervention, a between-subjects t-test, and repeated
measures t-test will be utilized. The dependent
variable will be the self-advocacy skills which the sample will
gain, and the independent variable is the role-play intervention
implemented. The t-tests will be computed through the SPSS
software.
Sample (N = 300)
These are the expected results
After the intervention is implemented in the first group, they
will show an increase in self-advocacy skills. After the
intervention is implemented in the second group, they will show an
increase in self-advocacy skills, and the first group will show
that they have retained their self-advocacy skills from the first
time the intervention was administered.
I just need to know how to do the mean estimation. How would
I estimate the mean differences?