Second Life (SL) runs as an economy of sorts. The money people buy in SL is used to buy items made by other residents. Goods sold by a creator can be exchanged for real world money. For some people, Second Life is their real life job.
This means that free money in SL (as in a cheat code) is impossible to obtain. How people obtain or make money is varied and I will list a few of them.
The simplest way to get hold of money to spend in SL is to buy it. This means you go to the SL website (the same site you used to register in SL) and log into your resident Dashboard (same username and password you use to log in to SL). Once you have your page up, go to the Accounting option on the left sidebar and from that select Billing Information. With this page open, you select the Credit Card option and enter your card details. This does not upgrade you to Premium Membership, but it does allow you to buy SL money using your card. This also allows you access to 'Adult' classified areas in SL. Once this is done and confirmed, you can go back into SL and buy money. Look for the L$ symbol at the top of the screen. When you buy money it will show you what it will cost you in US Dollars. If you are not a US resident, use your browser search engine to find an exchange rate converter to see how much that amount of Dollars will cost in your own currency. Currently, around $4.20 will buy you L$1000. SL will never take money from your credit card without asking your first. Once you have finalised your transaction, you may need to relog for the money to appear.
Be aware that on the Dashboard page there is a section called 'Recent Activity' that are links to forum posts. Recently, there has been a scam trying to get people to buy L$s through a forum post which spams the forums. Do not follow these posts at the promises of a better money deal. You part with your cash and get nothing in return, plus your credit card details are compromised. Always buy your L$s through the symbol at the top of your screen while you are in SL.
There are a variety of jobs you can do in SL to earn some cash, but all require your time and some even demand a financial outlay of sorts.
HOSTING: Many clubs need people to help 'work' an event. A host will send out group messages, welcome guests, add guests to the venue's group and prompt guests to tip the DJ/artist/staff/venue. Some of these places pay a set wage for a 2 hour event, while others only pay you in the tips you get from guests. A good venue will pay you AND allow you to have tips.
DANCING: There are a variety of clubs that need dancers. In General areas, you just dance and perform some of the functions of a host. In Mature or Adult clubs, your avatar may be required to wear an outfit or dance naked. In strip clubs, you may earn money the more your avatar takes off. SL dancers are required to look good, so you will need a good quality skin, hair, eyes and shape. You may also need your own wardrobe of clothes.
DJing/MUSICIAN: DJing is what I do to make money in SL. For this, you need an extensive music collection - at the very least, in your chosen musical field. You also need special software that is compatible with internet streaming software and allows you to create playlists. Some DJs use venue streams, but it's a good idea to have one of your own. The cost in music and renting a stream can make DJing prohibitively expensive for some and you really do need to have a passion to share music. If you can play a musical instrument, you can use the same music software and rented stream to perform 'live' in SL. Some venues pay you to play and all should allow you to take tips as well. I can make around L$5,000 doing a 2 hour DJ set.
CONTENT CREATOR: See all those nice things you want to buy? They're not made by the owners of SL - they're made by residents like you and me. That's why the quality of things you can buy varies so much. To make things such as clothes, you need to learn how to build with prims and to paint with something like PhotoShop. There is some financial outlay required here, as you will need to upload the textures you create (at L$10 each) and you will either need to buy or rent the space for a store to sell your wares. You may also want to purchase advertising in SL search.
As you can see, making money in SL usually requires spending on your part. You need to be passionate about what you do, otherwise you will stop having fun. My advice is to buy money until you decide what (if anything) you want to do for an SL job. I've only skimmed the surface here, because space dictates the length of my reply.