When it comes to Dark Souls the biggest thing you gotta work out at the start is what kind of build you want. All the starting classes are just just preset ups to give what ever you're after a head start. Since your new I recommend to experiment, see what types of weapons do it for ya. Also, very important, stat wise, never pour any points into resistance, its worthless, but vit and end is universally good for all builds, so never feel bad putting points into those. Once you have a rough idea what kind of weapon style you want (Dex for speedy weapons, Str for slow but powerful weapons, Int for spells, or some points in faith if you wanna use some miracles) pour points into the stats you need for your build. Also, on average anything above 40 and you get really small increases at that point, though many people will pump one of those to 50 or higher at times for whatever reason, so don't be afraid to do so if you want.
Upgrading wise you have to get tinitate shards, chunks, and embers for really high upgrades which you get by farming certain creatures, certain spots where they can be picked up, and certain npcs that sell some of the tpyes of materials. You can look up some vids or check the wiki to find out about the different kinds of upgrading paths what they need, but in general, upgrading is one the most key things in doing well in this game. You 40 str wont do you too much when your Demon ax is only +1.
Lastly, and most importantly, have patience. Seriously, if a you're getting your butt kicked by those monsters blocking that hallway, stop and check your surroundings, Take time blocking there hits and seeing when you should dodge, block, and hit back. This is super vital so learn it well, and don't be afraid to die a couple of times if it means you can master the monsters moveset and never be swamped by them again.
There's a lot more I can go on but for the most part these are the key 3 aspects that you'll want to keep in mind when playing. Good luck chosen undead.