Playing Old School Runescape is much more complicated than simply collecting OSRS gold and exploring the dungeons. First, you need to make sure you get the most out of your skills, especially your combat skills. Fortunately, there are ways to increase the combat level in a reasonable amount of time.
For starters, you want to hit level 10 as your first big benchmark. You can do it by using the chickens on the farm at Lumbridge. Since these are the early stages of a level up, you'll find it growing up pretty quickly, and you can also use feathers and raw chicken dropped by chickens to improve your cooking skills. This should help you reach level 10, after which you can go to the Lumbridge Swamp to deal with the Laws of Gaint found near the cemetery. This should take you to level 20.
To reach level 30, you now want to shift your focus to the level 9 NPCs known as Al-Kharid fighters. These require some preparation before facing, so bring some food with you if possible. Defending them means it's the perfect way to get to level 30. It can even be an AFK method to level up, as long as you have enough food with you.
For the next 40 levels, my advice would be to focus on Hill Giants. The reason they can get you this far is because of the amount of XP you can get every hour. Not only that, but their drops are worth a lot of OSRS gold. You can reach this area after purchasing a copper key on the Grand Exchange. After you get the key, go to the correct area and enter the cottage to start.
Reaching level 80 is best done with giant spiders. With that in mind, prepare yourself with food and drinks before diving in. Then go to the Stronghold Security Dungeon and go to the third floor. This is known as the Pit of Pestilence, where you reach an area where you can move back and forth so you can move forward well here.
There are several viable ways to reach combat level 99. If you want to train above level 80 it's worth going to the Corsair Cove dungeons. You will meet Ogress Warriors here, which requires some preparation. You will find that these enemies can fend for themselves defensively, but they also have a decent attack that you can fight against. It goes without saying that you are well prepared for it.
The methods described above are very effective when it comes to leveling the fight. It can be a bit tedious and exhausting to get the most out of it, but in the end it's more than worth it if fighting through levels is something you want to achieve.