As every language has sounds unique to it, so goes for how they're transcribed. For first time learners, this process of putting sounds to paper is crucial, since what they see on the page is is what they'll connect to those sounds. The words you're reading right now, for example, all have their sounds associated with them, and that connection allows people to grasp 'em, speak 'em and use 'em how they you see fit.
What does that mean for saying foreign terms through Japanese phonetics?
Well, let's say someone heard the word 先生(せんせい) for the first time while talking to their friend fresh out of Karate class. With nothing in front of them to refer to, they start rooting around their memory banks for any combination of letters to produce sounds close to 先生. This can create 'Sensey,' 'Sensee,' 'Sensai,' 'Cencei' or anything else that can fit the word's sound, and any one of the combinations can be the 'right' one if someone with enough authority and conviction in their answer says it is.
This authority is why, long after Catholic missionaries in Japan 1st used Roman letters to write out Japanese sounds-a method otherwise known as Romaji-multiple systems of it are still alive and kicking. Among them, the ones familiar to folks in and of Japan are the Hepburn System (the defacto one for modern students) and the Kunrei Shiki system (gov't made one used in Japanese schools and by older generation students).
The Kunrei Shiki System, in particular, can cause rough spots for those not used to it, since the Romaji it uses for ツ, シ, ジ, チ and others are tied to sounds in other languages, ツ using Tu, シ using Si, ジ using Zi and チ using Ti. Does that mean those sounds can't be made in Japanese?
Not where Tu, Ti and Di (along with a few others) are concerned. 'Tu' Sounds-like those in Tune & tool can be rendered one of two ways: 「ツ」 or 「テゥ」. 「ツ」-said the way you first learned it-is the common way Japanese-speaking folks say it, while 「テゥ」-said like the actual 'Tu' sound-is used when you want to get closer to its natural sound.
'Ti' sounds-as in those heard in Ticket and T-shirt-are much the same way, with 「チ」-said like you already know how to-being the common way and 「ティ」-said like the actual sound-being closer to the original version. With this and the other stuff you have-and will!-pick up, you'll be able to start putting the pieces together, like when your chats turn towards stuff like Baseball and you suddenly hear the word チーム in discussing the Giants and the Tigers.
In contrast, 'Di' sounds-like in Dig and Dimple-need just one way to render them: 「ディ」(which is said how you've always used 'Di' sounds);'Zi' sounds-used in Zip and Zim-are much the same, except they use ヅ, which is said the same way you learned at the outset. For 'Du' sounds-as heard in Duke and Doom-, you'll need to make your thinking a bit flexible, since they use two entirely different sounds, namely 「ヅ」and「デゥ」. For those needing a refresher,「ヅ」is said the same way as「ズ」、and is the preferred way to render 'Du' sounds, 「デゥ」being said the same as the 'Du' sounds you're been using all these years and best used when you want to get it across in a chat.
When to comes to typing 'em out, the computer knows that whether you type it as Chi or Ti, you probably want「チ」, and thus gives it to you on screen. However, if you want something like this (テゥ), you'll want to type in this order: Te(to produceテ), x, then your chosen vowel, applying a similar process get something like this(デゥ)-except you'd type de instead of te, naturally.
I can certainly understand if all this feels like playing guitar with your feet-I still sometimes confuse the the 1st part of Tunagaru with Tuna, myself-, but learning these quirks and systems will let you see the language in ways few students ever do, including how abbreviation of foreign terms work (coming soon!). Just remember, different systems don't always mean different sounds.
ñ There are 2 systems of Romaji: Hepburn and Kunrei Shiki
ñ 'Si' sounds are rendered using「シ」
(EX: Sims → シムズ /Sick → シック)
ñ 'Zi' sounds are rendered using「ジ」
(EX: Zipper → ジッパー/Zit → ジット)
ñ 'Tu' sounds are rendered using 「ツ」 or 「テゥ」
(EX: Tour → ツア /Two → テゥ)
ñ 'Ti' sounds are rendered using「チ」 or 「ティ」
(EX: Ticket → チケット /Tingle → ティングル)
ñ 'Di' sounds are rendered using「ディ」
(EX: Dill → ディール/Dip → ディップ)
ñ 'Du' sounds are rendered using「ヅ」 or 「デゥ」
(EX: Dooby → ヅビ/ Dufus → デゥファス)
As best as you can, render the words Tune, Dupe, Dizzy and City into Japanese phonetics
As best as you can, render the words チップ, シンプル and ジルチ into English phonetics