Once you have decided to quit smoking today, you have over come the biggest hurdle, as trying to quit without really wanting to, is not going to be easy. The decision to quit gives way to new questions: How to start, what can you eat/drink to help, how to lessen cravings... it's not really as difficult as you think; once you've made your mind up to quit smoking today, it's just like going on a diet. A little more difficult of course, as smoking is an addiction and like any other addiction, is very tough to break.
Try to set yourself a routine of diet and exercise and other aids, and stick to it. During the first 72 hours or so you will need to drink plenty of acidic fruit juice to help remove the alkaloid nicotine buildup in your body and to help stabilize blood sugar levels.
The best of these is cranberry juice, which will do the job whilst supplying your body with an abundance of vitamins and nutrients.
Fresh fruit and vegetables are a must, along with whole grains, seeds and nuts. You should be able to avoid gaining too much weight if you eat sensibly; weight gain is often associated with quitting smoking, this is because not only are you inclined to eat more to help stave off cravings, but smoking increases the metabolic rate and you will burn calories quicker, so obviously when you first quit you should try to adjust your calorie intake, at least until the metabolism returns to normal.
The best way to quit smoking is to take it one day at a time; follow an exercise routine, even if it's only walking, and find something to keep you occupied in idle moments. Many smokers agree that it's not only the craving for nicotine, but what to do with their hands when not working or otherwise occupied. Depending on your skills, try sketching (or just doodle drawing) knitting, writing; anything which keeps hands busy. Keeping a diary or journal is a good idea, as not only does it give your hands something to do, but you can keep an account of your progress; how you felt, how difficult/easy the day had been, things you had to eat or drink and how they helped, and so on. Open with "I decided to quit smoking today" and on each day re-affirm your commitment to quit.
You will definitely need something to snack or nibble on throughout the day, so arm yourself with plenty of healthy low-cal snacks: grapes and cherries are good, or cherry tomatoes (anything small which you can pick at individually) or sticks of celery are ideal, as not only will celery help calm cravings, but the body actually burns calories while digesting it! So there's a double bonus in eating celery.
The decision to quit smoking today was a difficult one to take, but having done so proves that you have strength of character and willpower, so you should be able to stick to your plan without too much difficulty. Trial and error will find the best way to quit smoking for you, but if you decide you need help, then that's okay too. There is always a solution, an easy way to quit smoking today.