You want to load data from an Excel file.

The xlsx package has the function read.xlsx() for reading Excel files. This will read the first sheet of an Excel spreadsheet:

# Only need to install once
install.packages("xlsx")

library(xslx)
data <- read.xlsx("datafile.xlsx", 1)

For reading older Excel files in the .xls format, the gdata package has the function read.xls():

# Only need to install once
install.packages("gdata")

library(gdata)
# Read first sheet
data <- read.xls("datafile.xls")

With read.xlsx(), you can load from other sheets by specifying a number for sheetIndex or a name for sheetName:

data <- read.xlsx("datafile.xls", sheetIndex=2)

data <- read.xlsx("datafile.xls", sheetName="Revenues")