First off all you need to get olympus gambit (2) and nerons contract (1) . Then you need to go to fast nerons contract (4) cuz this item is very good especialy giving you +20 move speed. After that you take royal seal for power and hp.
If your team is good and you are getting killing blows take psi-scimitar, if not then take entropy aegis for the hp and the shield buff.
Play safe with Shazam cuz his ulty has a cd of 120s and your other abilities can't make much damage if you are not feed.
Always be in mystic defens, initiate a fight with your Q then pres W for additional damage in this point your enemy will try to run away, finish him with R. If you didn't killed him, press E to slow him down and kill him with Q.
There is one thing you must know, Shazam is weak in early game so you need to play safe. It's good to go with blaster like Joker or marksmen like Cyborg, even with Poisson Ivy for her heals. Also going with Doomsday is amazing (if he know's how to play).
Hope you like this guide, good luck in the game