Go to the bathroom first, and most of the fecal matter will come out. If you want to you can take a small "anal douche" (~6-10 oz warm water enema). If you do that two or three times you'll make sure that you don't encounter anything until your intestines kick into gear again (you have a cycle, often connected with when you eat). Give yourself about a half-hour afterwards to let your system absorb the water and rest. Condoms and gloves also help.
Best start-off toy is a finger. You have a few, your partner has a few. Start with yours and some lube, and rub around the outside of your anus. Learn to enjoy the feelings, which requires that you psychologically _let_ yourself enjoy the feelings. Jack Morin recommends that you also look at your anus with a mirror.
Once you've done that a while you can start slowly penetrating yourself. A glove is a good idea because it makes your fingers smoother. Make sure you use plenty of lube, and go slow. Again discover the feelings and let yourself enjoy them.
Move up slowly from there. Maybe add a second finger, maybe get a small butt plug. Once you're comfortable playing with yourself and enjoying your anus you can bring your partner into it, but start small again - you'll tense up a bit because you're sharing control with someone else.