I agree with Shellz31 - starting with a small toy, even though it may not look or feel filling, is the way to go. Even better, start with a lubed up finger and pay attention to how your body responds. When you feel comfortable with your finger, move up to a smaller toy like a butt plug or anal beads or a slender dildo. It's all about working your way up to larger toys but at the same time paying attention to how you respond to the toy and the sensations that come with it.
Don't ever force anything - you will cause tearing and bleed! Be safe and have fun.