Ok, here's the secret: tactics, position, endgame, and opening are
not seperate entities. They flow seamlessly, and any divisions are
artificial.

Capablanca said that all of chess stems from an understanding of the
endgame. After all, the opening game is merely a tool to get to a
beneficial middlegame, and the middlegame is only a tool to either
checkmate your opponent, or get to a favorable endgame.

So too with tactics and position. Position is about creating tactics.
After all, what value is there to having your knight occupy a
beautiful post in the center of the board? It is valuable, because of
the tactical posibilities it presents.

And vice versa. A tough opponent won't allow you to gain a great post
for your knight for free. It'll take a tactical combination to force
him to give up that key square.

So to answer your question about what to study - study each aspect of
the game independently, and constantly think about how they tie
together. Study the Sicilian opening, and think about its positional
strengths and weaknesses. Think about the tactical opportunities it
presents. Think about the kind of endgames it produces, and how to
make those endgames in your favor.