Typical spreadsheet:

• rows: observations (typically individuals)
• columns: variables

A data.frame is the R equivalent of a spreadsheet.

Basic R:

• read.table("some_data.txt") for general text files
• read.csv("some_data.csv") for csv files

In packages:

• readr::read_csv("some_data.csv") for csv files
• readxl::read_xlsx("some_data.xlsx") for Excel format
• haven::read_dta("some_data.dta") for Stata
• …