Katakana is one of the three writing systems used in Japanese. Like Hiragana, Katakana is phonetic alphabet - each letter represents the sound of a syllable. Letters themselves have no particular meaning. Katakana and Hiragana (basic and advanced) represent exactly the same set of sounds. Basically Katakana is used for foregin words and names, and sometimes for mimetic words (gion-go/gitai-go).
Katakana was also developed by simplifying the form Kanji like Hiragana. Generally speaking Katakana letter strokes tend to be more straight while Hiragana is more curvilinear, because many of Katakana letters look exactly the same as a part of its original Kanji.
On the other hand, some of the Hiragana and Katakana letters are made from the same Kanji symbols. In this case, they have similar or almost identical shapes.
In my opinion, it would be easier to remember Katakana after learning Hiragana because they represent exactly the same set of sounds, even some of them have similar shapes. Then Katakana would be helpful when you start to remember Kanji symbols because Katakana letters are used in a part of many Kanji symbols.
Thus, I would like to recommend you should remember Hiragana, Katakana and Kanji in this order.