All you do is type in your topic of choice at the top, select what type of device you want it on and then narrow down your search by checking the boxes that fit your needs (free vs paid, games vs educational, etc.). Then a list of options will show up and you can start choosing ones to look further into that might well work for your students.
Once you find some apps you like, you can click directly on those and then read reviews, find ratings and get more information about that app. You can also click to find other similar apps or simply click the download button to be taken directly to iTunes. It's very user-friendly and takes you right where you need to go to actually get the app!
So, if you are looking for a way to find some apps specific to topics you are working on in your classroom, I recommend checking AppCrawlr out! I just love that I can type in "word families" and find a whole list of apps that might work well for my little friends! Happy searching :)
Have a great day!