Jensen Williams

My 1996 F-150 has the 5.8 and it has a high idle and it started doing it yesterday. My truck hasn’t had an O2 sensor since I bought it and it’s never had this problem till now. I’m not sure what’s the problem, I could use a little knowledge and assistance as I am only 16 and know practically nothing about the engine and it’s systems. It idles around 1400rpm while in park and 1000 while in drive. I drive this truck daily and it’s lived in Minnesota it’s whole life. I’m not sure exactly what I should do. What could cause this and what should I replace to solve it?.