Cleanse
Give yourself a good cleansing. Or, give a good cleansing to your bottom.
This can be as simple as a soapy finger in the shower, or as elaborate as a full enema.
For your first time, you may prefer to take care of this separately and privately, or you may want to go all out and share the experience as a sort of foreplay and exploration.
Whatever you decide, get yourself clean to whatever standards you and your partner agree on.