When you're getting ready for your first hike (or your thirtieth, for that matter) you have to consider how you will keep your belongings dry. Even "waterproof" backpacks might not keep out all the leaks (because the seams aren't always sealed, and this can quickly come undone) and waterproof pack bags are incredibly expensive, especially considering that they tend to wear out within a period of about 2 years.
Because of that, thrifty hikers have found a solution! Go to your nearest grocery store, and pick up a pack of trash compactor bags! These bags are thicker than your average trash bag, can be bought unscented, and can be easily repaired with duct tape! Most likely, you won't even have to repair them: they last many trips, and cost less than $1 / bag, so you can easily replace them without spending a lot of money.
Even if you don't plan to hike in the rain, it's great to keep the contents of your packs dry from rain, deep creek crossings, or leaking water bladders by lining your backpack with trash compactor bags. These bags are lightweight, cheap, and surprisingly sturdy — stronger than normal trash bags.
If you find it difficult to pack your bag in this "free space" method, you can use smaller trash compactor bags, multiple trash compactor bags, or ziploc bags all packed together into a trash compactor bag (over kill? maybe, but it works). Best of all, if you're going kayaking or canoeing, you can keep some air in these bags when tying them off, which will give them buoyancy should you loose a pack to the river!