There are quite a few trainer kits out there that come with like 3 or so different sizes, so that you can work your way up. I would start with one of those.
If it were me, I would get one (or one more to add to a kit) that has a skinny taper to use first, so you can get used to simple insertion, before trying out ones that start to stretch you on the girth end of things.
Digressing from there, there is a hard acrylic toy called Aneros that works like a plug but is actually meant for hands free solo play. You kind of do kegel exercises with it in and it's specially shaped to massage your prostate. I bring this one up because apart from getting used to penetration, you can also use this one to get in touch with the p-spot sensations you're ultimately after to get your hole nice and hungry for more and help the process along.
Good luck!