gray symbols mean not modernly used
The alphabet is made in a chart formation with the first five symbols next to the vowels mean that they are simply vowels and nothing else. The other symbols mean that they corrispond to the consconant and the vowel as well except for the single "N" which is used as a lone n and nothing else.
The Katakana alphabet is relitively simple is accordance to that a begginer should memorize a line at a time. The best way to memorize is to reapeat a line over and over again. First, copy a line. Second, (cover the copy line and the alphabet) copy the same line with help with your copy at times when needed till you have a good sense of them. Third, (cover all the copies) copy the line without any help till you have memorized it by heart.
Do this until you have mastered all 46 of the katakana alphabet.
There are about 100 katakana symbols in total but these are the base. Meaning that these symbols are used to represent the others like "da" and "po" which are not listed but use the same letters but with a dot or two dashes next to it.