This is a better question than I first thought because, whilst there's lots of guides and FAQs on this subject, they are disorganised, duplicating, and don't cover mobile apps specifically. I'll try to gather all the relevant information here.
What you need to attribute
Unless you are using CC0, pretty much all CC licenses require attribution. The best guide I found is CC's article Best practices for attribution, which provides the handy acronym TASL, or Title, Author, Source, License. There is no one right form of attribution, but as long as you include all four, you are good. For example, this is a very minimal attribution that still satisfies TASL:
Creative Commons 10th Birthday Celebration San Francisco by tvol / CC BY
Because it includes Title, Author, Source (linked via title), and License.
Unfortunately, there are a few details if you want your attributions to be perfect. For example, if you have modified the original, for versions 3.0 and up you need to note as such, and for version 4.0 the nature of the modifications. On the other hand, the title is not required for version 4.0.
How you should attribute
There's no specific guide on attribution in mobile apps, as these guides were written before the rise of mobile apps. There are different recommendations for online and offline attribution, the difference due to the ability to display working links. However, I'd argue that mobile apps are somewhere in between, because although you can display links, having those links be clickable may be bad UX. Let's look at the examples given in the guide Attributing Creative Commons Materials:
Books, journals and magazines
• Include the relevant attribution information next to the CC work or as a footer along the bottom of the work on the page that the work
appears on; and
• Alternatively, you can list the CC works in the back of the publication. If you take this option, it is best to list the works in the order in which they appear in the publication and indicate this to the reader.
Photos and images
• Provide the relevant attribution next to the photograph, or close by
(eg on the edge or bottom of the page) if that is too obtrusive.
• Include the relevant attribution information next to the CC work or
as a footer along the bottom of the work on each slide that the work
appears on; and
• Alternatively, you can include a ‘credits’ slide at the end of the
show, that lists all the materials used and their attribution details.
• Include the relevant attribution information with the work when it
appears on screen during the film; and
• If this is not possible, attribute the work in the credits, just as you
would see with music in a commercial film.
• Mention the name of the artist and that it is under a CC licence
during the podcast, like a radio announcement, and provide full
attribution on your website, next to where the podcast is available.
So you see there are a few common themes, which you can try:
Attribute right next to the CC work, whether that's along the edge, or in a footer on the same page. For mobile apps you want to be careful, due to the limited screen space this can be obtrusive.
Add a credits or about page in your app, where you list all the CC licensed works. Android itself provides a good example of this: if you go to Settings > About, there should be a section on open source licenses, and there is a lot. Note that unlike CC licenses which recommends links, typical software licenses require showing the license text in full.
Provide a link to a web page with full attributions. This is the least obtrusive option of the first two are unacceptable.