Pretty sure all the learners find distinguishing these two pretty difficult, as well as ン (n) and ソ (so) which I'm sure you'll encounter soon. Though, since ツ (tsu) is the equivalent of つ and I'm sure you know how it can also be used. Speaking from personal experiences, encountering it in cases like the ones below made it really easy for me to remember it: ・バッグ（ばっぐ） ・カップ（かっぷ） ・ハッピ（はっぴ） ・ラッキー（らっきー） ・クッキー（くっきー） ・... ツ was probably for me personally the easiest one for remember because of its usage. It's just a matter of associations in my opinion. I still have some problems with them myself since katakana isn't used that much overall (at least, well, compared to hiragana), so you probably shouldn't worry about them that much. You're gonna master them all after a while anyway. Well, if it does help (even though you've probably realized it by now) those stand together since they are bent in the same way, and it helped me at least (and it still does): ツソ (tsuso)　　シン (shin) I don't know man, this just helped me most personally, so I'm not sure how it's gonna be with you but I'm glad if it did help a bit!

I'd say you'll get the hang of it pretty quickly. Note that the long piece in シ is written going from bottom left to top right, so it's a smooth oblique line. On the other hand ツ's long piece is written in the opposite direction, so it usually has a steeper grade. The former is also thicker at the bottom left, and the latter at the other end because of the stroke direction. Also, the start point of all three pieces line up vertically at the left for シ, while they align horizontally for ツ. They are really very easy to tell apart once you get a bit more familiar with them. And here's a page that explain the above with a lot more detail: http://www.thejapanguy.com/learning-katakana-heres-a-cool-trick/

Look them carefully with the heart of a child. They look like human faces. The face looking up to the sky is シ(shi). The face looking down to the ground is ツ(tsu).

An example: シーツをこうかんする/ changing the sheets.　カタカナは　えいごが　おおいです。so you can guess. Don't worry that much.