Hey there @wolvy68! Welcome to Boundless! It’s good to bring a slingbow where available and a strong enough slingbow for the tier planet you are on just in case you need to defend yourself, creatures don’t currently spawn underground in caves, but occasionally mobs might fall in and roam the caves aimlessly.
For some good tips and information i recommend reading some of these:
It has quite a bit of information posted by others to help new players like you, i also have one in the list somewhere if you’d like a more in depth idea on tool usage, masteries, attributes and what kind of tools are what, but there is also lots of information in @Stretchious’ website he has built for the community to use in finding crafting recipes, what planets there are and where to find certain resources: http://boundlesscrafting.com/
@Stretchious is also working on a work in progress skill calculator, that you can find here: Boundless Crafting
It’s quite stable from my experience and is very helpful for planning out that perfect character! 
I hope this was of great help, and I hope you’ll enjoy the game Wonderstruck has created so far!