There are different ways and places to go but I have personally found a few to stand out in the past. Mintwallen, Hydras and Dwarf Guards are some of the best places in my opinion in that order. Most people will tell you that collecting loot is the best way and that's how I look at it. Loot bagging is an essential part of everyday Tibia and makes for a proficient source of income for any vocation. Just try to think of it as free money. Most people don't pick up little stuff like Hatchets but if you think about it, it's actually like leaving 25gp on the ground repeatedly, which deducts from what your gross total could be. That's my point of view but I don't blame you if you don't collect the little things usually anything under 50gp as can be a bit bothersome to go all the way to Fibula to sell them but keep in mind you should be collecting Bows and Cross Bows so you'll have to make the trip anyway. You can drag your bag around with you or hide it with at least a backpack of bolts as partial contents, having some extra backpacks makes looting more enjoyable and since you won't carrying it because you should have done the POSTMANS QUEST the weight should be of no matter. If you're not good at saving money like me, just save up the loot keeping it organized, which makes for a faster more efficient selling experience. I usually sell a loot bag that weighs at least 100k oz because at that weight your pretty much guaranteed a decent sum of cash.
People will always need help doing quest. You can charge them money to clear the quest or even accompany them. Here are just a few that occur often enough to note.
BLACK KNIGHT QUEST- The idea is to run the Black Knight around the room while you bolt it and use Sudden Death Runes, you can charge a group of low levels a set price and it adds up depending on the number of heads on the hunt. You'll need to find the key in a dead tree near by. (Need level 50)
BANSHEE QUEST- This quest is a bit more tricky as the last seal can't be infiltrated by people that have all ready done so, but the people always need help killing the Warlocks and just for support. (Need level 60)
STEALTH RING QUEST- This is a easy set up to be honest, I can go invisible and clear the entire pyramid with one cast of Invisibility. It's located to the far northeast of Darashia. (Need Invisible)
EDRON ORC QUEST- Pretty easy for the most part, just a lot of Orcs and one Giant Spider to deal with. It's located through the sewers in the largest apartment like complex to the south of the southern most Edron Castle gate. (Shield recommended)
BLOOD HERB QUEST- Another easy one being that a Giant Spider is the only creature that poses anything-immediate threat. (Shield and Heavy Magic Missile Runes recommended)
DEEPER FIBULA QUEST - This quest takes a little planning with the amount of creatures your killing. People in newer worlds will need help around level 50 as well as more with the progression of time. You need a key or two down here so make sure you do your homework and ask around. (You have to be at least level 50 to get through the doors so any higher just makes it easier)
People need money to advance in level, as they level they make money also. What I'm getting at is that you can essentially make money by capitalizing on this idea. I personally usually invest in Mages or Knights since their gain is usually higher than that of a Paladin. Let's say you loan a sorcerer 20 backpacks of manafluids, you could make a deal like you get all the loot he collects as payment which depending on the skill of the Sorcerer should bring you in a little award. The downside is the reward might be small but if you think about it and you can increase the amount loaned as time progresses to receive larger rewards and its free money.
If you keep the Trade Channel open enough you'll sometimes notice a nice or maybe decent item selling way below the average price. Your goal here is to buy an item for as low as possible then sell it for the normal price. Patient is a must in this venture as items are hard to sell sometimes. Keeping this in mind next time you see a Crusader Helmet for 4K don't think twice just buy it because at the worst case scenario you can have someone sell this item to a Djinn.
You can buy items cheaper in older worlds and this sometimes comes in handy. Think about it like this, if I was to buy a Magic Plate Armor in say Antica for 300K, then I could simply world trade it to a newer world where it's say 650K. By world trading a item like this you double your money. You can trade money from your current world to another to do this technique, but don't trust everyone now. It takes times and a few times getting tricked sometimes to learn, but after a while you'll be able to smell a trap from a mile away. Trade low amounts and you might want to be trying to find out about reputations of the recipient. There are plenty of ways to find this out ranging from asking around for local gossip or in the Game Chat Channel.