Tips for a first tattoo:
-Be really, really sure of what you want. This may seem obvious, but really. Don't get a tattoo if you're still thinking 'well this or that would be cool as well..' or if you're in any kind of state of 'I want a tattoo... NOW.' it won't do you well in the long run. ;)
Also keep in mind that the tattoo will stick with you for the rest of your life. So if you have an idea and you aren't 100% certain, what is the little time you will wait compared to the time you will actually have the tattoo? ;) (I'm currently waiting until I'm done with school and have a job to get a tattoo I really want. I'm in my first year and have 3 more to go after this)
-Look for a tattoo artist you like and makes work in a style that you like. Just because an artist can make realistic looking tattoos doesn't mean this person is good at what style you want your tattoo in.
-Make sure to work together with this artist. They often have an opinion about the design you want and can make it look more awesome and unique. Also they can look at the size of the tattoo and if that is doable/a good idea.
-Don't skimp out on money. This is a really important one as well, a cheap tattoo won't do you much good. Most artists work by the hour, you can sometimes discuss what the price will be on average when you show them what you want and what size.
-Don't start out with a big tattoo. Some places hurt more than others, and for some people it hurts more than for others. My mom got a tattoo on her upper arm in full colour, it was a bit like someone scratching just through your skin, annoying, but not that bad. I have two tattoos on my wrists (just lines, so 1 needle was used rather than several) and if felt like constantly getting tiny paper cuts.
-If you are getting tattoo'd, tell the artist if it hurts too much and you need a small break or a glass of water. They will probably set a small line first and ask you how the pain is. This is more about getting the tattoo rather than wondering what kind of tattoo, but you can find lots of info about this online ;)
These points may not really help you with what tattoo you should get, but I think that is a really personal thing. I can only advice you to start with something small, on which you might be able to build forward later, but still love on it's own.
Some things you could look at are chapter-art, different illustrations and quotes from your favourite characters. Perhaps you could also think of books given/read by your favourite characters or elements that made these characters special and were specific to them. (Luna's glasses, Dobby and his socks and hats, etc).
Save pictures from your favourite characters that somehow speak to you as well as quotes and the likes. Look for other people's tattoos from the series, or from others series and save them, even if there is just one tiny thing about them that speaks to you. Print them, cut them out, make a collage of the tiny things you love. Write them down and start piecing things together. That is at least how I usually manage to get down to the bottom of what I really want when I feel a bit overwhelmed by the choices offered to me. ;)