Mann-Whitney-U test using excel data on R programm

I have a question about Mann-Whitney test. Could anyone please help me to answer my following questions and I will be very grateful with your help. I have a csv (excel) with several thousands of samples whose RNA expression levels after different treatments, as following tables,

enter image description here

I want to compare the differences between two column each other for all column, (a1&a2, a1&a3, a1&a4,,,,,a3&a4), using R. Each column has over 1000 rows.

I already install R and can do easy calculation. But I do no know the procedure that will import the data from excel and calculate the Mann Whitney U test using R program. If possible, I want to get the result in table. Please help me.