The most time-efficient way to get Lindens is to do an RL job and buy Lindens with the proceeds. You can buy Lindens from Linden Lab at https://secondlife.com/currency/buy.php
One US dollar will buy somewhere between 250 and 300 Linden, usually closer to 275 or 290.
The most fun-efficient way is to develop an SL skill that people are willing to pay for.
Making stuff is popular - clothing, hair, furniture, animations, sounds, buildings - everything. This probably amounts to making your own job! Run a small business selling your products, or ask around finding people who want what you can make.
DJs are in demand, but do brush up on music broadcasting law. Skilled event organisers are also in demand, and NCI has a class on event organisation.
Dancers are common, but while some clubs that hire dancers actually want people to dance, others use it as a synonym for prostitute. Ask questions, and look at the environment.
Also, of course, there are plenty of jobs in the sex industry.
Some people make a good SL living in sales or administrative roles: land is popular, running a mall or estate or handling real estate deals for third parties.
You may be able to find a regular working-for-someone-else job in Second Life. Search->Classifieds often has jobs, and in forums.secondlife.com there's an employment forum.
These jobs are actual jobs: you're helping someone else, doing something they value enough to pay you for it. It's not like jobs in The Sims where your characters vanishes for a few minutes and comes back with money, nor is it like jobs in RPGs like World of Warcraft or EverQuest.
Choose a job that's fun for you - if it's something you hate, you may as well earn a much higher rate of pay doing something you hate in the real world.
Also, you can attend contests and other events - NCI has Show and Tell, Blitz Build and Blitz Makeover events that pay Lindens for winning. NCI isn't the only place that has contests.
Many NCI parties have a money-giver, that hands out 10Linden at a time, randomly to the people at the party. Some other places have similar parties.
If you are less than 30 days old (from your original rez date, not 30 active SL days), you can use money trees. In SL, money does grow on trees! To find them, click on the Search button on the bottom of your screen, select the Places tab in the window that opens, and type 'Money Tree' into the text bar in that window. (Then hit the Search button in that window.)
The many places that come up will almost all have money trees, and most money trees will explain how to use them. The trees have 'dollar bill' leaves among their normal leaves, and some fruit. Click on the bills or fruit to get money.
Finally, there is 'camping'. Camping is a system where you are paid a very small number of Lindens to stand around idle at a particular location and make it look busy.
Be careful you don't spend more in electricity, bandwidth and depreciation costs than you earn in Lindens!
To find camping places, use the same technique as for 'money trees'.
Still on the topic of money: avoid scams.
Pyramid deals, ponzi schemes, false banks: everything that you encounter in RL is also in SL. Gambling is outlawed in SL, as are banks unless they're regulated by some RL government.
In shops, beware of people who aren't actually the store owner, offering you 'deals'.
Some people make items that they give away for free. I'm sure you're not surprised to hear that some people take those items and sell them! Before you go and buy something, check to see what's available along those lines for free.
There are also people offering business-in-a-box kits. Everything you need to start your own business, including goods! The problem with these is that the goods are often available free elsewhere, and even if they aren't, you'll be in direct competition with everyone else who buys that business-in-a-box!
Keep your scepticism on, and you'll do fine.