All the advice above is good. Leave your ass alone for a few days, it will heal on its own. Try not to strain when pooping, and do not wipe vigorously, use a little water on some toilet paper maybe (no wipes with cleaning scents).
As with buttplugs, dildos come in a range of sizes. Start SMALL! Use LOTS of lube -- more lube than you think you need -- it's easier to figure out after you're used to things how little you can get away with, but don't start skimpy.
If it hurts, stop. Wait a little. Your ass is used to pushing out things much thicker than the starting size dildos or plugs you should be beginning with, so if you're hurting or bleeding that means you're starting too big or not giving yourself time to relax. The first few times you use a larger size, it should take you at least 5 minutes of stopping and starting again, always adding more lube. If you're going faster than that, you're just asking for trouble.
It'll get easier as you learn to relax. And your ass will stretch a little over time. But the key is TIME... You need to step up to the bigger dildos, and we're talking a few weeks of practice with smaller ones until you can expect anything to stretch. Your body is not made of SillyPutty that can stretch to ridiculous size at will.
Start small. Go easy. Lots of lube. If it hurts, stop. Wait till it goes in easy before you begin jamming it in hard. And if you notice bleeding, increase your lube. Some tiny amount of blood is not unusual but is a sign that you aren't ready yet for whatever you were doing that caused it. So don't be afraid to back off.
What you don't want is to give yourself an anal fissure or a severe hemorrhoid, which can take weeks to heal or might need a small surgical procedure. So go easy.