Generally speaking, there are two kinds of R in American Accent: The Light R, where the /r/ comes at the beginning a word or before a vowel, as in rose, red, around, and the dark R, where the /r/ i comes at the end of a word or after a vowel, as in are, air, ear, or, her, poor. The quality of their production is different.
The are two ways to pronounce the R correctly;Try pressing both sides of your tongue against your molars (the bottom of them, not the top), then hold your tongue rigid in the middle of your mouth. There should be a noticeable space between the roof of your mouth and the middle of your tongue.
The other option is to do the same as before, except move your tongue farther back in your mouth (kind of “bunching” it to the back).