get a transit app
for directions and to know when your bus will arrive.
Transit is a good one.  Moovit
is pretty fine, too.

get a TAP card
for easy fare payments.  TAP info

get a map
a paper map and compass, or an app like these:
Gaia GPS - free offline map ideal for dayhiking. additional maps available with subscription.
AllTrails - great trail guides and info. subscription required for offline use.
Organic Maps - free offline route planning and track recording. open source, no account required.
Hiking Project - free offline map and trail guides.
Komoot - very limited free offline map, additional maps available for purchase, no subscription.

get knowledge
Hiking for Beginners:  essential hiking tips
What to Bring on a Day Hike
:  water, snack, sun protection, etc
Hiking 101:  the basics of mileage, elevation and routes
Hiking Glossary:  all the lingo

get more trails
12 Hikes on Foothill Transit
Modern Hiker

Hiking Project
LA County Parks Trails

get involved
Nature For All:  promoting transit to nature
Trail Angeles:  connecting forest volunteers