The starter decks have a good mix of cards and are very evenly matched. They should give you a good idea of where to start building your deck. Locations are important, and the last thing you want to happen is to be caught short, especially at the beginning of the game. Due to deck construction rules, you must have a minimum of twelve locations in your deck (four of each of the three types), but I recommend at least 20 in a 60-card deck.
The Mystery Machine will probably be in almost every deck. It was an integral part of the show and will be an important part of most strategies, but do not rely on it alone. You will need Friends and other items to be able to pull off Locations with high Fright Level. The Fright Level is compared to the Bravery of Mystery, Inc., to determine the outcome of a turn and is the ultimate determining factor in success or failure.
The Villian cards are some of the most fun to play. Even the low cost Villians can be powerful in conjunction with each other or other cards such as events. Some Villians have special abilities taken from the show. While others try to use brute force just to scare you. I like the Vampire Bat. It raises the fright level by one and forces your opponent to lose Scooby-Snacks. Anything that makes your opponent lose Scooby-Snacks has to be good.
The biggest (and most subtle) part of the game is resource management. You only have a set amount of Scooby-Snacks to spend per turn. If you use them all while doing your own investigation, you will not have enough to stop your opponent during theirs. You have to walk a fine line. Do you worry about your own investigations or stopping your opponents? I have found so far that a balance attack works best. Remember your turn consists of investigating and them trying to stop your opponent. If you can manage to do both with the limited amount of Scooby-Snacks, you are well on your way to winning the game.