Play a character you would actually want to play rather than what might be "needed" at the time. There is nothing more frustrating than leveling a new toon and then realizing you don't necessarily enjoy what is required to play it. Remember, it's you purchased the game and it's ultimately your money every month.
Worry about number crunching after level 60. Honestly, levels go by so quickly from questing now that fine tuning and tweaking your character is next to pointless unless you plan to do PvP in a particular level bracket. All your equipment will be replaced over and over again until it's time for the end game (post level 80).
Find mods and add-ons that work with your play style. Ask yourself some questions - do you go on more raids, do you quest by yourself frequently, do you like a high level of organization, do you like customized interfaces? Many problems and issues have already been addressed by add-on developers to help you enjoy your game time a bit more.
Hardcore raiding works for some, a more casual approach is better for others. It's all about the amount of effort you would like to put forth in developing your character(s). There is no wrong way to go about it - whatever works for you is fine.
Figure out what kind of player you are by the amount of time you have to spend playing World of Warcraft. Knowing what style suits you will be helpful in choosing what circles of people you play with. Once you find folks who share a similar viewpoint and play style, you'll find your experience far more enjoyable.
Improve your overall playing experience by immersing yourself in World of Warcraft lore. There are many sites out there who publish bios, plotlines, and other extraneous information.
Contrary to popular belief, guilds are not mandatory in World of Warcraft. Far from it - a person could occupy themselves with solo-oriented content for years on end. Joining a guild is a personal choice which should reflect on the sort of play style you have and/or would like to have. Consider your own motives first, then find a guild that matches them closely.