Not to be redundant, but obviously you should start with the most natural toy there is, your own body. I think that any dildo with a relatively small circumference (try something between 4-5 inches) is the next best option. When selecting a size, go with something smaller than the circumference of your own penis. I really like the realistic dildos because they give a little while inserting. Dildos made of hard silicone (like this prostate massager) are too much for my backside. However, I think this toy is great for beginners. I personally would like it to have stronger vibrations... I want to feel the toy inside me, not just poking into my prostate. I plan on getting the we-vibe salsa to insert into this toy for stronger vibrations.
To sum everything up, I would start with YOUR fingers, you know your body better than anyone else. Play with different fingers/positions and when you feel comfortable move on to bigger and better things. I know straight men might be skittish to try a dildo, but a suction cupped dildo works wonders in the shower. You don't have to "ride" the dildo. Just having pressure on your prostate will give you a much stronger and enjoyable orgasm.