Two things to remember about sound design; you are enhancing the story and characters, and the sounds are often representational rather than reality. That means that the sounds you use are not the sounds of the objects themselves. One of the "jokes" in the sound design community is: for a pistol use a rifle, for a rifle use a shotgun, for shotgun use a mortar, for a mortar use artillery, for artillery use a tank, for a tank use a nuclear explosion, for a nuclear explosion stick a mic out the window of your car and drive at 80mph. The lesson here is that once a sound is mixed into the sound track (btw, all audio elements together - dialog, Foley, sound FX, ambience and music - is the sound track; the music is score) the sound is quite often not "big" enough.
Many times a sound effect is really multiple sounds layered on top of each other. When doing a car crash, for example, I'll use a car crash sound from a library, then layer in other impact sounds to enhance the emotional impact, tearing/crunching metal sounds, breaking glass (another btw; "glass" isn't really glass these days, it's polycarbonates that don't "tinkle" when hitting the ground, but real glass sounds are used to enhance the experience) and whatever else strikes my fancy. All of these sounds are processed by EQ and various digital effects such as reverbs, delays, etc. When doing Foley, for example, I've used strips of paper and tape from old audio cassettes to be leaves rustling in the wind or for footsteps. On one film a character was walking on RR tracks. I used a 3" diameter steel pipe (simultaneously recorded with three different mics) and shoes with hard rubber heels even though the character was wearing sneakers.
Sounds can provide a lot of information about the characters, location and plot if used properly. This is most effective when discussed during pre-production.
There are lots of free sound effects out there; most are junk but there is also some really nice stuff if you have the patience to weed through all of it. The LJudo library has some very useful sounds, and the Freesound Project is a terrific resource. Just keep in mind that some of these free libraries require you to give proper credits; a small price to repay for all of the time and effort of the folks who made the sounds.
Another great resource is filmsound.org. Lose yourself in there for a few days and you'll start to get a handle on - and hopefully get excited about - the possibilities of sound and sound design. Especially read "An Open Letter From Your Sound Department" "An Introduction to Film Sound" and "Designing a Movie for Sound ".