That question is hard to answer specifically without knowing what you need to do. In general, you can select rows that meet a condition with the filter() function from dplyr. You can select columns with select() from the same package. Base R has the subset() function that allows choosing both rows and columns. Base R also has the [ and $ functions for standard subsetting. The slice() function and its associated helpers in dplyr are another route for choosing random rows or rows that meet certain criteria.
What do you need to do?