My car idles very high. Both at a stop and I can hear/feel it while I'm driving. What do I need to replace?

It's a 2001 Kia Sephia. This issue is also causing more gas to be used and it's a 5 speed, when I let go of the gas it makes my car jerk from the rpms being too high. Is it the idle air control or something else?? Please help! Thank you in advance. I just can't afford to take it to a mechanic.
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