Hey Taichi,
There are a bunch of y+ calculators online, like this one from cfd-online or this google spreadsheet. These are reliable but I do recommend that you make your own spreadsheet for y+ evaluation.
So basically these calculators take a desired y+ as input and return the thickness of the first cell. It gets a little bit tricky here though, because a lot of solvers, SimScale included, calculate the solution at the cell centroid. So you should multiply the calculated first cell thickness by 2 before taking it to SimScale.
Just to give an example, let’s say the calculator returned a value of 0,002 meters for the first layer thickness, If you input this value to SimScale, the solver is going to calculate the solution at the cell centroid (0,002/2 = 0,001 meters) which will result in a y+ that is too low. By multiplying 0,002 meters by the factor of 2, your first cell centroid will be correctly placed at the desired 0,002 meters mark.
Hopefully I could get my point across 
/Ric