If you are a non-exclusive zoophile, I would actually suggest an experienced and considerate human partner. Come clean with that person that you would be having your first experience, and you don't really know what to expect. Ask him if he wants you to do anything special to prepare (some guys demand enemas, which you can get cheap at a local pharmacy). Be considerate, BE ON TIME, dress up in something casual but CLEAN and nice, be prepared to have some conversation and a cocktail in order to relax, be forthright with that person about how open or NOT open you are to the use of drugs because even most of them that use drugs are considerate to non-users, and I suggest specifically requesting protection. Once he starts, don't try to be a hero, but tell him exactly how you are feeling at any given time. Feel free to talk--along the lines of "I like that" or "How far have you gone in?" or "May we try another position?"--during sex: it's actually classy, and it's worth learning how to do it well.
If you are an exclusive zoophile, that's okay, but fair warning, dogs tend to take an approach of "poke until it pops in somewhere," and you'll just have to make peace with that. Dogs are great for being enthusiastic and passionate lovers, though!