I have found in the past when I have done anal on those very few occasions that it gave e the urge to go to the toilt from that end.
So firstly I suggest go to the toilet, then have a shower to completely clean yourself out.
Like WildIrish say's it does have alot to do with attitude...hence try not to think about to much I know it will be hard but it does help.
Get you partner to start by lubing up one finger and inserting it into you and finger it in a circular direction, when that feels confortable continue with the next. Depending on the size of your partner will depend on how many fingers he will get up to.
Once he sees that all is fine he lubes up, for the 1st time I recommend that you do use a condom even though people think it makes the penis biiger yes to a point but at the same time it is lubed and very smooth so it make it alot easier for you partner to slide in....

Good Luck!!!