Typically, you would compute your effect size after you have completed your study based on statistical estimates of, for example, group means and standard deviations.

The free program G*Power (https://www.psychologie.hhu.de/arbeitsgruppen/allgemeine-psychologie-und-arbeitspsychologie/gpower) allows you to conduct "sensitivity analyses" in which you can compute the required effect size given fixed alpha, power (1 - beta), and sample size.

The study is already completed ? In that case, you can just calculate the relevant effect size statistic.

And bear in mind that standardized effect sizes are not always the best choice.

Article Standardized or simple effect size: What should be reported?