If you're curious to start fisting you'll need to spend some time preparing. You can do this with a partner or alone. Choose dildos or safe objects of 2 or 3 different sizes to work up from small and comfortable to a medium stretch. You're training your sphincter to open instead of close when something is pushed into it. Then once you can do that, using plenty of lube, you begin to train it to open wider than before by slowly upsizing the dildos.
Just relax when you do this. Choose a rhythm that suits you. Any sudden movements will provoke a tightening reflex. Once you've trained for width you can begin to train for deapth. This can be quite enjoyable, but always choose smooth, safe objects with no sharp edges or ridges. About 6 inches inside you will find your second sphincter which is where the fun really starts. It is normally closed, but if you have had an enema or douch beforehand, your bowel will be empty enough for this second ring to open with some training. This leads to the lower intestine which allows another 8 inches of penetration before winding it's way up into your gut.
For those who have been following at home we now have about 14 inches of potential penetration. You will not achieve this for quite a few months. As explained above, you can practice the initial stretching by using your own fingers and eventually your own hand. Other things you can practice are gaping, which involves the conscious control of your sphincter contractions and gripping, by which I mean holding something like a ball or piece of fruit or water inside until you want to release it.
As you can see, up to this point Fisting is not so much about losing control of your muscles but GAINING voluntary control of them.
Socially, fisting is a minority fetish, so don't expect that your partner will like the idea. If your relationship is open however, you may develop a circle of fisting buddies outside your love relationship. Or else your partner may take the chance to learn something new, as well as something new about you and possibly themselves.
In big cities there may be places to meet FF lovers. Otherwise use the Internet. It would be a good idea to look at quite bit of Fisting porn, both commercial and amateur, to first see if you like the whole idea. You'll find amateurs on XTube.com and commercial porn from Fisting Central and Club Inferno as well as other fetish sites like Dickwadd.
Fisting can be very intimate, just between 2 people or else you can hang your ass in a sling in a club for all to use...it's up to you. Some people go through phases when they need a fist and then they don't. Be sure that you want to start FF because after you get good at it, a small to average cock will probably not have the same effect anymore, but that's why we're here on LPSG isn't it?