Most simply put, fasting is giving up food to spend additional time with God. Scripture gives examples of partial fasts (like Daniel...who gave up breads, sweets) and full fast (it is speculated that Christ endured a full fast, water only). Likewise, scripture gives us examples of fast lasting anywhere from a single day (sun down to sun down) to fasts lasting 40 days.
The length and type are between you and what God places on your heart. And the fact you are asking, let's me know God has placed this on your heart. Fasting is NOT about weight loss, it's about purifying your body, mind, soul. It's about dedicating time to God. Christ himself said, "WHEN you fast..." not if you fast, but like me it looks like you are having to learn for yourself. It isn't something that most churches talk about on Sunday morning....but it's there, in black and white.
Most often when we see fasting in scripture, we find someone who has troubles ahead or something that are unable to conquer on their own. They show God that they are able to discipline themselves and lean completely on him. And yes....it works. Fasting seems to draw you near to God in ways that prayer alone doesn't. Therefore, many people suggest fasting when there is something in your life that only God can conquer (sin, addiction, illness). But I have found that fasting solely for the opportunity to worship and draw near to God is equally fulfilling. When you draw near to God, he draws near to you.
As for dos/dont...most people would say someone should complete a lengthy full fast (i.e. water for 30 days) the first time. I would be inclined to agree, unless you know that God has placed this on your heart. He will bring you thru what ever he asks of you. Also anytime you fast...especially for longer than 1-3 days, it is VERY important that you don't jump at the first burger you see once your fast ends (work you way back into solid foods). And most importantly, do not use fasting just to loose weight, it then looses its purpose, and you will gain it ALL back the second you resume eatting.
I'm no guru....but feel free to email me. Someone I would recommend if you just want extra research is Jentezen Franklin ( JentezenFranklin.org ). It seems to be the call that God placed on his life, and he leads his church in an annual fast. He is merely a Christian man/minister, but the presence of God is clearly upon him.
In the meantime, I am praying for you. God will bless you if you decide to move forward in this discipline.