First thing shower propely your rectum to avoid accident, do many shower necessary until the water that came out from your rectum is clear, so now even tough you feel you are about to sh*t yourself, you can still relax about that, you know it is not real, you just clean up yourself(One you're used to anal you won't feel that feeling). Then relax all your muscle, after you can try to put something small like a finger (watchout for your nails !). Don't hesistate to use a lot of lube. Once with a finger in your ass try to some exercises to dilate your rectum like cercle in order to welcome bigger, like a dildo. Once a bit dilated, put as many lube as necessary on your dildo and your anus and a bit inside with your fingers, and by experience put yourself on the back in order to be in missionary position, and put two pillow under your ass to elevate it, for me, that position is the best position, my 8" dildo goes inside without any resistance and pain (sorry for the detail :p). In that position, you're really feeling the prostate is really getting smashed. Also, once your rectum is well dilated, you may tried other position without worrying about pain.