If you have the pleasure of owning your dog at a young age you have the honor of exposing them for their first time to the bathtub with a guaranteed positive association. If perhaps, you've adopted your dog their reaction and association with the bathtub is a gamble - but aim to make it positive for them!
First's things first - You (yes you!) need to be prepared:
Be sure that you're wearing clothes that you don't mind getting dirty or wet. Additionally, you'll want to move all the grooming materials into the restroom (treats, a favorite toy, fur-brush, shampoo, conditioner, at least two big absorbent towels, a non-slip tub mat and a large cup for rinsing).
Time to set your dog up for a successful bath:
Coax your dog into the bathroom with you and shut the door behind you. There is nothing more frustrating than catching a wet dog running loose in the house. Once you are in the bathroom, you'll want to brush their coat and get rid of all their shedding fur (If you own a dog with hair - I'd suggest seeing a groomer before hand). After a good brushing you'll want to coax them into the bathtub (even if that means you're getting in there with them or if someone needs to be there to help). If your dog is not good at "stay" or "wait" I'd suggest having them on a short leash while someone holds them during the bathing process. Every step is a big step - so let loose on the good praise and treats each time it's time for their bath.
Now that you're both ready, let's introduce the action:
Turn the water on to a gentle flow out of the faucet (rather than the shower head), and make sure that the water is luke-warm. Let our dog stand in the running water to get familiar with the feel. You can reward them with at treat or perhaps by introducing their toy into the water and making it more of a game. Once your dog is comfortable - you can turn on the showerhead and allow your dog to react. Once they are familiar with the water flowing out of the shower head I'd have your dog "sit" in the tub or stand calmly as you bring it around the side of their body to get them wet. I have always avoided putting it directly on their head. Once your dog is wet, open the shampoo and let them smell the soap. Once they have a sniff lather them up with suds and give them a thorough rinse. If you'd like you can repeat the process with conditioner.
You will want to make this experience positive and exciting for them. So remember to give them plenty of praise, treats and patience.