How can I prepare for a rapid chess tournament (not chess sites tournament!)? For example, in one and half months, I am going to play a 15+5 tournament. I tried to find some rapid strategy or some advice on opening or middlegame whether I should play positional or tactical or attacking. However, I couldn't find anything. Could you give me some advice or strategy for 15+5 time control?
The best thing to do is play some games with that specific time control so you can get used to the pace.
Whether to play positional or tactical is a decision around your skills and what you know about your opponents play. Though it is considered more difficult to defend than attack and that can lead your opponent to use more time. It is also true that finding a plan is time consuming; so I would just play toward your skills. Certainly you should study a lot of tactics so you can calculate quickly.
In general, you don't want to fall behind on the clock. But you shouldn't panic if you do. And don't play on your opponents time pressure. 5 second increment is a lot. You should get comfortable defending "drawn" endgames where you can make a move within 5 seconds.
Semi-dubious openings like the Smith-Morra Gambit are common in quick tournaments. Prepare your openings so your opponents don't get to have any such fun!
There is no special rapid strategy for rapid. You should play the game you play in standard time. If rapid is new to you, you should play some games with the shorter time to get used to the faster moves.
But no rule without exception: If your opponent is stronger than you, you might try to make things more complicated to give him a chance to blunder. You might blunder, too, but that you would do with any move rate. The faster the game, the sharper the lines you can choose. (According to Simon Webb, Chess for Tigers)
Exception to the exception: If you play with time increment, that will reduce the chance of the stronger opponent to blunder. The higher the increment, the more normal lines you should choose.