Don't be put off -- installation is really quite simple. First you need the physical space to place the new HDD -- they're standard size and fixing holes, so look inside your case to make sure there is a vacant space. If there's only a bigger space available (usually for CD or DVD drive), you can buy spacer brackets so that your HDD drive will bolt in properly.
Then, as you say, you need a y-lead for the data and another one for the power. You may already have the data lead in your PC because its not really a Y -- just a second plug crimped onto the ribbon lead 1 to 2 inches away from the end of the lead. You should already have spare power leads within your case -- most power supplies come with more than you need for a basic setup.
Then, once the HDD is in, you might need some help in formatting it. But post another thread and I'm sure someone will assist.