How can I know how to write Japanese sentences?
In Duolingo you select from boxes/multiple-choice for the correct answer. I can usually tell from looking at the answers, but how can i remember for typing and reading? Does this come in later?
There is an option to switch to a keyboard in Duolingo already on the bottom when you have the word bank exercises, but to do this you need to have a keyboard already installed, which should be easy if you have a mobile device or a newer computer. (You have to go under the keyboard settings on mobile or the language settings on the computer, and find the Japanese keyboard. You can search how to do it for your specific device and program by googling it). For typing on the computer you basically just type Romanji and select choices from a drop-down menu, using “x” in front of vowel and “y” sounds to make a character smaller if needed. In my system (Windows 10 on PC), I sometimes need to select if I want to type in the Latin alphabet, Hiragana or Katakana, but on my IOS devices I can use a Hiragana keyboard and Katakana options will still show up, so if you’re using different keyboards for each device be aware that they might operate differently.
I can usually tell from looking at the answers, but how can i remember for typing and reading?
With time, and after you've done enough exercises and reviews to grasp the sentence structure and accumulate vocabulary, you will start to know how to construct the correct answer without relying on the boxes, and you will be comfortable switching to keyboard typing using the Japanese input methods as Keith and DragonPolyglot already explained.