Step 0: Card Acquisition

You probably have a few cards already, but if you want to start building better decks you will need more.

The cheapest way to get new cards is to find someone to give them to you. It might be awkward but most players have a lot of extra cards and they will jump at the chance to get rid of them. Just go to a local game store and ask the players if they have extra cards they can give you. If someone says no, don't worry, you'll almost always be able to find someone trying to get rid of extras.

When you're just starting off, I would caution against buying cards online or trading with more experienced players. Once you are more familiar with what cards you want or what cards are worth, these are best ways to get new cards, but until you are comfortable with Magic prices and have a firm idea of what cards you want, there are a lot of potential risks.

Most players are ethical in trading, especially with newer players who are not familiar with card values but there are people who will take advantage of your lack of experience and try to cheat you. If you are going to be trading, I would recommend preparing by looking up the prices of the rares you have on a site like magiccards.info as well as some of the cards you're interested in. Don't be afraid to walk away from a trade if you feel the other person is trying to cheat you or if you disagree with their valuations. Players will often overstate the value of their cards and minimize the value of yours to try and obtain a better trade.

Buying cards online or from stores has fewer risks of being ripped off because the market will generally force stores to charge fair prices for their cards. You also have the option of comparison shopping, which i highly recommend. Some stores have better prices for commons and uncommons while others will have better prices for some rares and worse for others. The big danger in buying online is buying cards that you don't really need.

This goes along with the deck building section, but you should try to be conservative in your purchases. A good idea is to use a program like Magic Workstation to test your deck ideas before buying cards. It's much better to figure out that your deck has trouble achieving its goal or beating other decks for free on the computer than after you have already spent money buying cards. Printing proxies of cards and putting them in sleeves along with a real card can help you get a feel for how your deck will play for a much lower investment than buying the cards.