You are just 18 and marrying at this age is too early. If you do not like that person, just explain it to your parents about it or you can directly tell it to that guy himself. You are at the age where you have to go for education and pursue your dreams. Find your passion and work for it. Love or arranged, both depends upon the individuals based on their understanding and love towards each other. Don't confuse yourself dear. For now explain your dad and convince him that you do not want to marry that person. Go for college and get your degree. You will definitely find your true love at the right time regardless it being love or arranged.
Do what you want my dear and make sure you convince your parents also. Wishing you a great life.