How the sales work
Every year, Steam has four sales. These are during fall, winter, spring, and summer. During these sales, most games are between 25% to 90% off, leaning towards the higher side. Steam makes these more interesting than your average sales by adding in flash sales, community choice, and daily deals. The flash sales change every eight hours and make several games much cheaper than before. The community choice sales allows the community to vote on which games get a larger discount next. These are essentially the same as flash sales, but the community gets to pick which games are included. Lastly there are the daily deals which change every day and include games which valve thinks should get a larger discount. Any games which are not included during any of these are most likely at least somewhat on sale.
How to prepare for the sales
Before each sale, there are several things you should do to prepare. These include adding money to you account, linking a credit card, and adding games to your wishlist. Obviously if you plan to buy cheap games you are still going to need some money. I recommend adding $60 to your account. This can vary depending on how many games you want, but it is always better to have more in case you do need it. You can get steam credit in two ways. The first way is to get it directly from your credit card by going to store.steampowered.com and logging in. Next you press the amount off money you have in the upper right hand corner (this could be $0.00 if you have never added any). Next, on the right hand side, click "Add funds to your steam wallet". From here you can select how much you would like to add and what credit card you want to use.
The second option is the option I have always used. You can go to a store such as Game Stop or Target and on the gift card rack they might have some steam gift cards. Grab as many as you like and buy them. When you get home go to the steam client and click "Games" in the upper left hand corner then go to "Redeem a steam wallet code". Scratch off the back of the gift card and enter the code.
You also have the option of just paying with you credit card directly. Be warned that if you add a credit card, it will require 7 days before you can make purchases with it due to steam's security, so DO NOT ADD A CREDIT CARD IF YOU NEED TO USE IT IN THE NEXT 7 DAYS!!! Now that you have some credit, you can go to your wishlist (Store -> wishlist) and add any and all games that you want. Make sure that the games you want most and/or the most expensive games are at the top. I will explain why later. An additional benefit of doing this is that if any of your friends want to give you a game, they have a clear list, but also when a game in your wishlist goes on sale, you get an email. After this you are all prepared for the steam sale to start.
How to get the best deals during the sale, along with some free games and moeny
Now that you are all prepared for the sale, it is time to find the best deals. Before you go out and buy all the games you want on the first day, you should know to only buy a game if it is part of a community choice, flash sale, or daily deal (now to be called "quick sales"). Never buy a standard sale unless it is the last day. This is because the quick sales are almost always the best deals you can get. Frequently the standard sales will go further on sale as one of the quick sales and you will be really angry if you bought the game as a standard sale. During some sales they give out free games if you are randomly picked after crafting a badge. I will not say specifics because it changes for each sale, if it is even there. This however leads into how to make some money.
During many sales, there are cards you can earn by voting on the community choice, buying games, and a couple other ways. You can either craft these into badges for a small chance to win some games and to display on your profile, or you can sell them to people who want to do the first one. To do this you go to the community market and click "Sell". I recommend selling the cards at the same price as the current low. This is for several complicated reasons that I will not explain now. During previous sales, one card can sell for $0.40 and with all the cards you get, if you don't craft a badge you can get a pretty decent game for free!
Hopefully this guide has helped you to get some great games at low prices and earn some money to! If the guide did help be sure to share it with your friends so they can get cheap games to.