The letters じ and ぢ stand for or represent one and the same sound. To be scientifically exact, they represent not a sound, but what linguists call "phoneme", but let's not go into too much detail, and think that they simply represent one and the same sound. 

Now, let's define the sound represented by these two letters as /JI/. Then, your question can be paraphrased as follows: When is /JI/ represented by じ and when by ぢ? How do I know when to use which letter to represent /JI/?

Notice, now, that what you are asking is very similar to such questions like: Hey, in English, there are many letters that can represent the sound /k/, for example "c" and "k" in the noun "cook".
The simplest answer is: There's basically no perfect rule, you just have to memorize the orthography.

However, knowing lexical etymologies and so called "phonological rules/tendencies" can help in many cases. For example, the sound /k/ in the word "introduction" is represented by the letter "c", not "k". Why? It could be explained in the way that the word "introduction" is derived from the verb "introduce", and in English there is a certain rule, or more exactly a certain tendency that the sound /s/ in a verb ending in "-use" becomes /k/ as a result of the deverbal nominal derivation. Similarly, why do we represent the sound /z/ in the word "housing" with the letter "s", but not "z"? I could give you a lot more examples, and explain how all these problematic sound changes / orthographic conventions came into being "sociohistorically", but you've already got the idea, right?

し = shi like in "she"

ち = Chi ( tchi ) like in "cheese"

ち is considered to be on the "T" line of hiragana. When it comes to using katakana, Japaneses use チ instead of "ti" in japanized foreign words, and ティ for "ti" in actual foreign words

oh sorry you mean じ and ぢ、ぢ is barely used both are the same sound. It is used when there are following sounds. It's similar to ず and づ = つづく = to continue ちぢむ to shrink

I also read that formerly, ぢ、was transcripted zi and was slightly different. It was used for kanji pronouced ji after another Kanji. So the word Kan ji that comes from chinese Han zi was written with that zi but it had been dropped. 

thank you so much :) that helped a lot OG1001 

thank you, i think i understand now :)