If you want to change all character variables in your data.frame to factors after you've already loaded your data, you can do it like this, to a data.frame called dat:
character_vars <- lapply(dat, class) == "character"
dat[, character_vars] <- lapply(dat[, character_vars], as.factor)
This creates a vector identifying which columns are of class character, then applies as.factor to those columns.
Sample data:
dat <- data.frame(var1 = c("a", "b"),
var2 = c("hi", "low"),
var3 = c(0, 0.1),
stringsAsFactors = FALSE
)