Edit
Using nano
(simpler):
# Open file
nano file.txt
# Save file
CONTROL + O
ENTER
# Exit
CONTROL + X
Using vi
(powerful):
# Open file
vi file.txt
# Search for "string" and go to line
/string
# Enter insert mode
i
# Exit insert mode
ESC
# Get the : prompt
SHIFT + Q
# From : prompt, save and quit
x!