There are tons of way to make money on BadKush. lets start with a few of my favorites.
Slayer- There are plenty of slayer options to make money but the best has to be abby demons. You need 85 slayer to kill but they drop Mystery Box and Abby Whips fairly often and is good money.
Boss Slayer- Boss slayer is my personal favorite method to make money but you may not have the gear/levels for all bosses. But making money off drops while getting slayer xp is pretty nice.
Crystal keys- most Npc's if not all drop crystal key half's for a beginner i would recommend crawling hands at slayer tower. Now what do i do with my crystal keys? you can use them at crystal chest at home to get great rewards! or you can play the safe route and sell to other players.
Thieving- of course thieving is always a option for money making. its simple there are 5 stalls at home. always steal from the highest level stall possible. when inventory is full sell items to merchant to the west of the stalls.
Hunter- Hunter is a great way for money making and really simple. buy a butterfly net from skilling shop, and head over to skills teleport and find hunter. go around and catch all the imps you can (stick to the highest level imp you're able to catch for better loot and experience) you can sell the junk you don't need to merchant at home. when you get to dragon and lucky imps that is where the money is. dragon imps have a chance to drop a Dragon Hatchet. Lucky imps drop clue boxes! you can sell to other players or open to obtain a clue either easy, medium or hard!
Vote- vote for vote points! you can spend your points in vote shops to get great equipment to use or sell to others! its easy money!!!
Event boss- i saved the best for last! There are events held every 2 hours, you must deal 50 damage to boss to obtain reward. which is a boss chest key. you can get some of the best items in game from this chest! You can also sell keys to other players!
Raids - Raids are one of the best ways to get drops, they take a few people but its worth it! Typically Range or Mage is used here and you need fairly good gear.