A full in-depth (not really) guide to get good at PvM (Player vs Monster).
Step #1: Finding Your Preferred Combat Style
Runescape has 3 different combat styles, magic, melee and range. Currently as it stands melee is best dpm in the game, followed by range and magic after that.
As a new player you'll start with level 1 in all skills (Assuming you skipped the tutorial), as you progress through the game and build your character in Gielinor you'll find yourself using all 3 styles to get Level 99 (Halfway to 99, is level 92. You can also level to 120 with no extra rewards for a sweet cape).
You will more than likely end up doing multiple quests during your time in Gielinor and will end up using all 3 styles as a result, as you progress through the levels however, you will unlock better weapons, spells, arrows etc. Do note that you also need to level up the Defence skill, as well as Attack, Strength, Range, Magic, Prayer, Constitution and Summoning to further increase your combat level which is calculated using the levels from each of the skills listed.
Get to know each combat style, and complete recommended quests too, certain abilities are locked behind quests which are 100% worth doing.
Step #2: Gear and Weapons
Assuming you've now found your preferred combat style, it's time to start looking at gear and weapon progression, you can find out what weapons you can equip from the skill guide, as shown below.
Once you've chosen your style, you'll likely end up referring to this guide to see which weapons you can equip, you will be UNABLE to equip any weapons that requires a level you have not yet reached, even if you are stat boosted.
Note: Check the defence skill guide to see which armour you can equip for your style, your attack, range, magic level alone will not decide what armour you can equip.
Again, I would highly recommend using the PvME discord for any advice on gear.
There are also a lot of items that will give you an extra boost or trigger certain abilities which will benefit you during your time in Gielinor, like the ring of death, ring of vigour and plenty more.
Step #2 1/2: Questing, Overloads, and Curses
So now you have your gear it's time to move on to something that is almost a requirement for most bosses, you will now need to do quests and skilling to unlock items and abilities that are SUPER helpful when bossing.
Curses are a different type of prayer, almost every person on member worlds use curses, assuming they have them unlocked. To unlock curses you need to complete the quest "The Temple at Senntisten", however this is a fairly long quest line for a new player but it's highly recommended by most players. For mid-high level bossing, you will need at least level 95 prayer for curses, level 99 for curses obtained from a Praesul Codex, dropped from Nex: Angel Of Death.
Overloads are a type of potion which will boost your stats by a certain amount depending on the type of overload you are creating, you can create these potions by levelling up Herblore which I recommend getting to level 99 in (120 if possible). This process can be expensive but is overall worth doing.
I recommend doing skilling during double xp when they come round as it speeds up the process a ton.
There are a lot of quests which are recommended to do in the game, whether it be for the rewards or the story, personally I'm not a fan of questing but it's very much doing the quests that reward you with abilities or access to certain areas, sometimes but rarely needed for bosses. You can find a guide that will detail the recommended quests for pvming which I recommend you take a look at.
Step #3: Practice
Like all games, practice makes perfect. You won't be a top tier god at bossing right off the bat, mechanics for some bosses can be a pain to deal with and you should get to know the bosses when you feel at a level suitable to do so, I would also recommend that you join a clan or a friend chat which will help you in bossing, Gielinor has tons of clans across all the worlds available that will take on newer players to help give them a boost in the game.
Bosses on this game can be hard, don't be stressed when you die learning a boss (Especially telos lmao). As I mentioned, practice makes perfect. You'll get it down with time, enjoy the game as much as you can instead of getting angry with mechanics or death costs (Oh yeah, have fun paying money when you die to reclaim your items lol).
Good luck, World Guardian.