First, I expect that you should at least know the basic difference (other than the writing) between the two characters: И is a soft VOWEL, while Й is a CONSONANT.
In terms of the pronunciation, и and й are pronounced as [i] and [j], respectively, i.e., й can only be a short-timing sound of и, due to the fact that it is considered as a consonant. You may refer to the following webpage to check for the sounds of different Russian characters:
Except for some foreign words like йо́гурт(yogurt), йо́га(yoga), Йе́мен(Yemen), etc., Russian words do not start with й, and it basically follows mostly after a vowel. On the other hand, in order to soften a consonant, Russian either places a soft vowel, such as я, ё, ю, е, and и, or the soft character ь, right after that particular consonant.
I hope that the above information gives you some idea between the difference of the two Russian characters. Good weekend.