Try this and make sure there are no errors. If it all runs, you're in good shape! Should make three plots, and install rnaturalearth, which is pretty cool

geospatial_test.R

#'------------------------------
#'
#' Script to make sure everything works
#' for you for the 2019-01-22 Geospatial
#' Data Carpentry workshop at UMB
#'------------------------------

# Make sure we have rnaturalearth installed
# If you do, comment out the install.packages line
#or just don't run it.
install.packages("rnaturalearth") 
library(rnaturalearth)

## STOP HERE - THEN RUN THE NEXT LINE ON ITS OWN
## AS IT WILL GIVE YOU A SERIES OF PROMPTS TO WALK THROUGH
install_rnaturalearthdata()


## OK - AS LONG AS THAT WORKED.....
# The SF library and ggplot
# If you get a package not found error, try typing
# install.packages("PACKAGENAME")
library(sf)
library(ggplot2)
library(rgeos)

#get the countries of the world
countries <- ne_countries(returnclass="sf")

#plot them
ggplot() +
  geom_sf(data = countries, fill = "lightgreen")    

#### Run to this point, then stop and make sure you have a map,
#### Then....try what is below

#A raster test
library(raster)
#Test a raster
rst <- ne_download(scale = 50, type = 'MSR_50M', category = 'raster', destdir = getwd())

# load after having downloaded
rst <- ne_load(scale = 50, type = 'MSR_50M', category = 'raster', destdir = getwd())

#aggregate - this might take a while
rst_agg <- aggregate(rst, fact = 50, fun = mean)

# plot 
plot(rst_agg)

#### Run to this point, then stop and make sure you have a map,
#### Then....try what is below
rst_df <- as.data.frame(rst_agg, xy=TRUE)

#Plot shapefile and raster together!
ggplot() +
  geom_raster(rst_df, mapping = aes(x = x, y = y, fill = MSR_50M)) +
  geom_sf(data = countries, color = "black", fill = NA) 

#### Run to this point, then stop and make sure you have a map

# Make sure these libraries load with no errors
# If you get a package not found error, try typing
# install.packages("PACKAGENAME")
# at the prompt and following any instructions - 
# and if there are any errors, email us
library(rgdal)
library(rasterVis)
library(remotes)
library(tmap)

need to install "rgeos" before you can run countries <- ne_countries(returnclass="sf")

ggplot() +
geom_raster(rst_df, mapping = aes(x = x, y = y, fill = MSR_50M)) +
geom_sf(data = countries, color = "black", fill=NA)

also if you have problems with plot(rst_agg) because your figure margins are too large, you can run par(mfrow=c(1,1), mar=c(0,0,0,0)) first