Since you've never tried double penetration before, you should start off with a toy that is slim. It's important to keep in mind that your boyfriend's penis will be penetrating you, and so that's why you will want to use a slender toy in either your anus or your vagina.
It's a very filling sensation and that's why you want to start with something small - you need to be comfortable. You also need to find a comfortable position so that you or your partner can control the motions. Make sure that you use lots and lots of lube. It's also important that you don't switch holes. That can cause an infection.
