If you are just starting out, and learning how to play magic, one of the easiest formats to play in is standard format. Standard format usually involves the last two main blocks released by the Magic: The Gathering manufacturers, as well as the latest core set Main Blocks with the exception of early blocks come in groups of threes that have the same basic theme behind them. Cards that are from older blocks are not allowed, unless they are reprinted in the new blocks, such as basic lands, which are always printed in every block.
If you do not have enough cards from the last two released blocks to make a 60 card deck, plus a fifteen card sideboard, or if you are not satisfied with the cards you have, you will have to buy some from the new blocks. One way to do this cheaply is to go online to auction sites and look up lots of “commons and uncommons” cards. These are usually around 25 to 30 dollars for a block that contains four times every card that is a commons and uncommons of one release of the three from a block, this will usually include a few basic lands if you need them. These blocks are a good deal as the commons and uncommons usually hold a few treasures along with a bunch of decent filler cards. A simple deck can be made with these, especially if you have a few boosters to throw in.
Another good option to obtain cards is to buy a whole booster box, This will get you four copies of almost every uncommon and uncommon along with a lot of rare and mythic cards. This will allow you to construct a decent deck just with what you have.
If you wish to have a more competitive deck, you will defiantly want to purchase a couple more common and uncommon blocks from different sets. Try to keep them as new as possible as they will not rotate out of standard quickly. Which means you will have more cards to work with when the next block comes out.
Remember when constructing your deck to keep it simple, at least until you get the hang of it. Play around with friends before you go to competitions. Try and figure out a style you are good at playing, some people like to go with a certain color, or two. Others like to mix one color with another. Try not to use more then two colors when constructing a deck, as your basic land supply will be hard to control.