While at it: relax, enjoy and take your time, don't hesistate to tell your top when you're in pain and want him to go slower/stop.
In order to properly relax (and as a preparation in general): clean yourself before. eating some high fiber food will help you balance your interior and help you become clean more easily.
you can also try to explore your bum on your own first with fingers and toys fitting. You shouldn't be putting small objects in, because they might not come out easily, also try not to insert anything hollow (like bottles) because it can break can cause lethal damage.
at least that's my 2 cents.