sudo -l
then i run the scripts mentioned by dachef
pay attention to suid files
check the weird ones on gtfobins
pspy to see what crons run and get a general idea if there is a way to root from there.
Generally from what i have seen so far the easy and medium linux machines are a mix of service misconfiguration, loose perms on executables, config files and folders, left-over keys or plain text creds in normal and backup files as well as sudo permisions on various executables.
Resources you could use:
IppSec youtube channel
The hard linux machines (at least the 3 of them that i have done so far) are of the same mindset but require way more enumeration and ‘back and forth’ to gather more information. Also their exploitation techniques are usually more a bit more advanced.