Almost as fun as being there is getting there. There are 3 aspects to the preparation for the trip: physical condition, equipment and logistics.
Unless you have loads of time on your hand that allow you to ride at your leisure and let the Camino take as long as it may, physical preparation will probably be the single most influencing factor in making it an enjoyable experience.
The contrary is true when it comes to equipment. Less is more. Weight will be your enemy.
The logistics of getting there and back will depend on your preferences and the size of your wallet.
While fixing a flat on day 3, I met a character who didn't take a vacation in 5 years. He saved up 4 months worth and set off on foot from Holland to Santiago. That's an option. But most of us have never had 4 months vacation nor will we ever.
Realistically there is 2 ways of going about this:
a) You decide how much time you're going to invest and then work backwards based on the average daily distance you can cover.
b) Choose a spot on the map and work out how many days its going to take.
For most a) will be easier. Time is usually the variable you will have more or less fixed and everything else will be planned from there.
Once you've decided how long it's going to take its time to get hold of one of the guide books. Try and get the Camino de Santiago 2005 by Antón Pombo. It's practical, has the best maps and covers all the routes.
Now you'll have to estimate how "fast" you can travel so you can decide roughly where you want to start. You're best off starting from a place that is easy to get to such as a big town or city.
As I've said before, weight is the enemy. The less you take the better. Obviously you don't want to go to the extreme of jepordizing your chances of finishing. Nor do you want to be unnecesarlly uncomfortable. But within those limits the less you take the better.
For clothes you want everything light, sweatwicking and fast drying... that means no cotton. My recomendations are on the "cold" side meaning that its just enough not to feel cold most of the time but at some points you will. If you're always one of the first in a group to feel chilly you definitely add "one layer of warmth" to the suggestions.
Die-hard "cold adicts" are the only one who won't also and thet extra layer if your traveling in winter and going over the pryrenees.
My trip was in October and I rode in temperatures as low as 2ºC. Look at the detailed stats here. This is what I actually used and did fine with. Out of ignorance I took more than this on the trip but just took it for the ride.
There is still a lot more to come for this page. You can jump to a page that will some day properly linked from here where I've posted the stats of the trip.