Since a picture is worth a thousand words, let’s have a look at the following chart to see how each one of the first 5 katakana is written.
ア(a): is fairly easy to distinguish as there aren’t really any other katakana which looks like this one.
イ(i): is a bit more tricky as it can easily be confused with エ。That is because エ looks like a capital “i" from our Roman alphabet. This is why it is very important not to make association to the Roman alphabet when identifying a katakana (unless it can help memorization purposes).
ウ(u): is very similar to another character: (wa) ワ. The only difference is that little vertical bar on (u) ウ which is missing on (wa) ワ. I had trouble differentiating both characters for a while until I figured out a little trick which I hope will prove useful for you too.
Interestingly enough, the hiragana version of ウ is う while the hiragana version of わis very different from its katakana counterpart ワ. But what really helped me to distinguish both characters is the presence of that slightly horizontal bar on top of the う which is somewhat transcribed in the katakana version: ウ as the small, horizontal stroke. In other words, both the hiragana （う）and katakana（ウ） version of “u" have a small stroke on top of them which is missing on the katakana （ワ） and hiragana （わ） version of “wa".
エ(e): as already mentioned before, エ can easily be confused as an “i" sound since it is very similar to a capital “i". The best advice I would give a beginner would be to remember not to be tricked with the similarities and remember that エ stands for “e", not “i".
オ(o): is also thankfully unique as far as its shape is concerned. It can hardly be confused with other katakana characters. It is also fairly easy to remember as all you need to do is to draw a cross with a third branch at the bottom left.