The difference is in voicing. If a sound is voiced, it means you can feel your voice box vibrate if you place your fingers there.
- /tɕ/ ㅈ is unvoiced. Always found at the beginning of a word, and sometimes in the middle.
- /dʑ/ ㅈ is voiced. Always found between vowels or voiced consonants and never at the beginning of a word.
The Fluent Forever Korean pronunciation videos have native speaker examples and detailed linguistic explanations. You should study all four videos, but I'll highlight the parts relevant to this question: