There are several reasons why you should use a sound card interface program. While using a simple patch cord between the mic/speaker connections WILL work, there are many reasons why this is not desireable or even safe for your computer (and possibly yourself).
Sound Card Isolation interfaces can be as simple as a couple of audio isolation transformers inserted into the line between the mic/radio and speaker/radio jacks, to complex DSP circuits of considerable size. Even if you build your own keying circuit, at LEAST put isolation transformers between the radio and the computer.
Commercial Interfaces are cheap insurance. They often contain opto-isolators, and other sophisticated circuitry that would cost you more to buy and build than a pre- built unit costs.
These are made by several companies that have provided outstanding support for Amateur Radio in the past.
When time and money permit, I will try to purchase at least one of each (West Mountain makes several varieties). These all appear to be good units, and I have heard on-air reports of satisfied customers of all the above, and most importantly, I have never heard a single negative report on any of these units.
There are many internet sources of do-it-yourself and kit interfaces. I will try to post some of the better ones from an information standpoint, (I have not built any of these kits). As always, solder connections are critical, check for bridges, and cold solder joints. Use a magnifying glass (yes, even if you're young). It could save you from burning up a card, or even your computer or radio.
One good thing about building your own Interface is you won't have to throw away that useless Windows CD full of nag-ware.
Sometimes the illusion is given that such a sound card interface only works with a certain operating system, (usually Windows, simply because that is whiat most people use). Actually, the sound card interface is HARDWARE that is supported by software such as Windows, Linux, Unix, MacOS, etc. As long as your operating system has good software and soundcards, serial and/or parellel port support, your interface couldn't care less which operating system you prefer.
If you think you may end up taking your sound card interface on emergency services/communications, or field day, etc.. think about whether you want one that requires external power, and of course size and weight will be a factor. Also, do you want one that needs to be connected to a com port?
Another consideration is cables. No matter how good an interface is, don't put the hand-held mic/speaker cable into the jack while the antenna is connected. I found this out the hard way. A simple solution is to buy a cheap camera tripod, make a bnc/coax connection that will screw onto the tripod screw mount, and put the antenna at least ten feet from the computer and radio. This will reduce the rf that gets near your computer, and particularly with laptops, rf will kill a computer quick.
And now for the disclaimers. I do not represent any of the companies listed on this page, I just find them to be reputable and worthwhile companies IN MY OPINION. Your mileage may vary, but I think you will be pleased with any of these units. But remember, nothing is perfect. If you are running serious power, take serious precautions.
I have seen a computer monitor that was not connected to a radio, (or even turned on for that matter), go up in smoke when bodacious amounts of power was applied to an antenna with a high SWR.