Welcome to the forum. You can do a land/water setup and you can keep fish in there too. Just be sure to pick a frog species that likes water habitats.
I keep fire belly toads in a 10 gallon tank that is roughly 70 percent water in surface area. They prefer the water and spend more time in it than they do on land. The water is not deep enough to keep fish, but if I had a bigger one, I'd try to put fish in there that the fbt's could also feed on.
Take a look at what kind of frogs and toads are available for sale in your area. Then search for care and habitat guides on the internet and make sure that is what you want to commit too. Mixing different types of frogs/toads in generally not recommended.
Once you've decided what you think you want...... browse through some of the posts in what ever sub forum matches your pick. If you are sure.. then build the viv first and get it established before you buy the critter. If it does have a large water feature and/or live plants, then let all that get stabilized for two or three weeks.