Look at reference materials. It may seem obvious but it's really important to know what dolphins look like. Use the Internet to look at images, watch dolphin clips on sites like YouTube, visit the nearest aquarium, look at books and magazines featuring dolphins and familiarize yourself with the subject of your designs. Sites like Photobucket, Wildlife Art Company and Sea Pics contain lots of excellent dolphin imagery to strengthen your work.
View tattoo designs (called flash art) at tattoo shops in your community. Most tattoo parlors have lots of flash designs and samples to peruse. Look at tattooists' portfolios in these shops, too, to see what other people have done. See what works and what does not. Talk to tattooists while you're there and get valuable information about what styles might work best. Traditional styles of tattooing might work for some designs, but maybe a more realistic design might hold some appeal for you as well.
Look at tattoo galleries online. Numerous websites provide lots of pointers on tattoo designs and contain large image galleries of samples. Everyone has differing styles and tastes, so it is valuable to expose yourself to a wide range of ideas and images so you can come up with the most effective design possible. Sites such as Tattoo Tips and Tattoo Design can be very useful.
Brainstorm. It may seem simple and obvious, but a straightforward brainstorming session with lots of sketching can get surprising results. Most great ideas are generated in this way and you might surprise yourself with how creative an off-the-cuff doodle session can be.
Have fun. Dolphins are known as playful creatures, and making art based on them should be similarly free-flowing. The creative process can be enjoyable, and a happy frame of mind is conducive to smooth and clever artistry and craft.