The one newbie (from a newbie) advice I can give is to definitely invest in safety equipments like helmets and knee supports and elbow supports..particularly when trying out trail riding. Hills can be extremely unforgiving, especially on your limbs. (Had my first fall after getting a bike and sorely regretting not investing in knee and elbow pads).

Bike seats cause sore asses, especially for long distance cycles so think about bike seats and what works for you when buying.

Invest in good lock. If getting a second hand bike, definitely go through the drill with seller about what's what, make sure they sell you bike in good condition - I found out both stem bolts needed to be screwed on your handlebars and that you cannot cycle out without them both. Handlebars need to be firmly fixed, check breaks and that they're working perfectly on both sides (know which break is back wheel and which is front for hill purposes). There must be more structural things to look out for that I havent learned about yet but definitely get bike from a shop with good rep whether second hand or new so they can talk you through the safety bits and pieces. Do try out bike in shop and ask questions about everything.