You mean not 'デ' but 'ヂ', don't you? 'デ' is the katakana for 'で'. Basically you use 'じ' for the dull sound of 'し' and 'ぢ' is the one for 'ち'. For example you write 'はなぢ' for 'nosebleed' because the word 'はなぢ' consists in 'はな(nose)' and 'ち(blood)'. However there are quite a lot of exceptions. Like 'じめん(ground surface)' is not 'ぢめん' although it should be 'ぢめん' because it consists in 'ち(ground)' and 'めん(surface)'. In addition, some Japanese don't care the difference of them nowadays and they tend to use 'じ' even in the case they should use 'ぢ'. It is better for you to learn 'じ' first and then leran exceptions bit by bit. For your information, the difference of 'ず' and 'づ' is similar to this case. 'ず' is the dull sound of 'す' and 'づ' for 'つ'.

Actually, sometimes it's confusing even for native Japanese speakers to decide which one to use. Good news for you is that most words are written with じ／ジ. In other words, ぢ／ヂ usage is quite limited especially in beginner-intermediate Japanese vocab level.Here are some examples of words written with ぢ／ヂ that you may come across with. 縮む（ちぢむ） 縮める（ちぢめる） 鼻血（はなぢ） 身近（みぢか） ご飯茶碗（ごはんぢゃわん） チヂミ etc. There is same problem for ず／ズ and づ／ヅ. Fortunately again, most words are written with ず／ズ. Rare exceptions are: 小包（こづつみ） 片付ける（かたづける） お小遣い（おこづかい） 続く（つづく）・続き（つづき） 手作り（てづくり） 三日月（みかづき） etc.

ah, thanks! :D I looked for it but couldn't find it, but now I know! <3 arigatou

Btw, that's ヂ, not デ. On computers, you will get ヂ by typing Di. (But the pronunciation is JI, as you know. )

Sorry, I really don't understand your question.