cd <path> changes the current directory to <path>
ls lists all files in the current directory
rm <file> deletes <file>
These commands should suffice for your purposes.
apt-get stores downloaded packages in /var/cache/apt/archives so your best bet is to navigate there and get rid of some. Go to that directory first:
cd /var/cache/apt/archives
Then use the rm command to delete some you don’t need. You can use rm -r ./* if you are certain you don’t need any files in the current directory.
There’s plenty of shell tutorials available if you need anything.