There is no precise "symbolism" in the foods we eat or do not eat while fasting, if by symbolism you mean this particular food stands for that particular virtue, vice, practice or whatever.
Rather, the foods which we are permitted to eat are those that were considered plain and unappealing in the fourth century Mediterranean culture where the Tradition originated. Fruits, cereals and vegetables were basic staples; shell fish were considered peasant food, to be foraged from around the edge of the ocean.
The things that made life worth living back then were wine (usually sour, and usually well watered), cheese (Mediterranean people tend towards lactose intolerance, so dairy products were eaten in the form of cheese), and above all, olive oil, which was used in the same manner as butter and was an essential ingredient in cooking. Fresh fruit are good in season, but even in the Mediterranean, not much was available during the Lenten season. Boiled vegetables, too, were not very appetizing. Meals, therefore, were plain, boring, and unappetizing. And that was the point.
As for what we don't eat, there were two approaches to meat: the first that it was extravagent--only the rich could eat it with any regularity; the second that eating meat (or any animal product, including fish and poultry) involved shedding blood, and that was seen as an undesirable passion. Shell fish, on the other hand, were simply "harvested" (cold comfort if you are a clam). Amphibians, reptiles and so forth were not eaten, but let's just say if you're wondering about them, then you aren't approaching the fast with the proper frame of mind. This is not about ritual purity, but about mastery of the passions.
As to how to begin the fast, I suggest talking to someone who can be a spiritual father to you. The best approach is not to do it all at once, but rather to set for yourself a goal and to stick to it. The goal should be what you think you could achieve--and just a little bit more. So, for the first year, if you have never fasted before, try avoid meat (and staying away from wine, beer and other alcohol should not be too much of a stretch), maybe allowing yourself to do so on weekdays if this is becoming too much of a burden. Next year, do the same, but fast more strictly on Wednesdays and Fridays, avoiding all dairy products. The following year, avoid both meat and fish, the year after add dairy products to the list, and so forth.
The point is, you should not go through the day thinking about what you will or won't eat that day. To keep your mind off of it, set yourself some prayer goals, and also attend at least some Lenten services. Each year, add to your rule of prayer, and be sure to give alms as well.
