When you open a terminal you are placed in your home directory. A few essential navigation commands:

pwd          # print working directory
ls -lh       # list files with human-readable sizes
cd /etc      # change into /etc
cd ~         # return to your home directory
cd -         # return to the previous directory

File and directory operations:

mkdir projects           # create a directory
cp file.txt backup.txt   # copy a file
mv old.txt new.txt       # rename / move a file
rm unwanted.txt          # delete a file (no recycle bin!)
rm -r old_dir/           # recursively delete a directory

Use man command to read the manual page for any command, e.g. man ls. Press q to quit the pager. Many commands also accept --help for a quick summary.

man ls