When beginning to learn to visualize, the best thing to do is start as simple and as slowly as possible.  Give yourself plenty of time to master one technique before moving on to the next.  There is no rush.  My suggestion is to practice each exercise for 15 minutes once or twice a day for a week each, starting with Exercise 1 and working your way down each week.

 
Exercise 1
 
Look around, out a window, at your desk, wherever.  Find one view and take it in for a few seconds.  Now close your eyes and recreate that view in your mind, as you just saw it.
When you are able to do this, hold your hand up in front of you and look at it.  Now close your eyes and see your hand in your mind.  Now, without actually moving, see your hand in your mind forming a fist, and opening up again.  You may need to actually watch yourself do this before being able to visualize it.  
Look out a window, at a tree or the road.  Watch for some kind of movement, whether it's a bird flying by, the wind in the leaves, or a car driving past.  Close your eyes and see the scene, and repeat that movement in your mind.
 
Once you can recreate what is directly in front of you in your mind, you are ready to move on.
 

Exercise 2
 
Hold your hands out in front of you until you get the image in your mind.  Now close your eyes and see your hands.  Think of your favourite fruit and visualize it in your hands.  This may take some time.  Visualize the shape of the fruit, and feel it in your hands (it sounds odd to think of it this way... it's partly a matter of remembering what it felt like the last time you had it; you do this without thinking about it all the time).  Now, in your mind, bring the fruit up to your nose and smell it.  Recreate the smell of the fruit in your visualization.  Should you feel so compelled and daring, bite into the fruit.  Recreate the taste of the fruit, the texture of the flesh, the juice.  If your fruit is large enough, look at where you've bit into it.  Examine the inside of the fruit - smell it, feel the juice dribbling down your hands.  Stick with this visualization as long as you like (you may find yourself going through several baskets of visualized fruit).
 
Once you can visualize what you know, you are ready to move on.

 
 
Exercise 3
 
Close your eyes and visualize your hands.  You should be fairly good at this now.  Now, create your own fruit.  It can be any shape you like, any colour, it can be fuzzy or smooth.   Give it a smell.  Make it smell like violets, cinnamon, vanilla, even motor oil if you're that weird.  Give it spots, stripes, whatever you like.  Turn it around and feel it in your hands.  Break it open or bite into it if you like.  Smell the inner flesh.  Are there seeds in it?  Is the flesh soft or hard, smooth or stringy?  Is there a lot of juice?  Spend as long as you like with this, and if you get frustrated, stop and take a break.  There is no rush.
 
Once you can visualize what is not real, you are ready to move on.
 

 
Exercise 4
 
Visalize yourself in the room you are in now.   Now, change your surroundings.  Maybe you're sitting at couch in front of a wooden coffee table (this isn't what you should visualize, it's just a suggestion of where some people might do this activity).  You might start by changing the colour of the table's wood.  You could change it to pine, maple, cherry - you could paint it white or black.   Change the wood into stone, clay - if you like, upholster your table!  Visualize the walls changing colour.  Mentally paint them blue, green, yellow, black.  Change the floor.  Turn it to carpet, hardwood, tile, even gravel.  Feel each change under your feet.  Change the material of the couch - make it a different colour, make it softer, turn it to leather if you like.  Visualize the smells as things change.  If you change the floor to soil, smell the richness of the soil.  If you change the couch to leather, smell the leather.  Eventually, visualize your entire room changing completely.
 
Once you can visualize new surroundings, you are ready to move on.
 

 
Exercise 5
 
Visualize yourself exactly where you are now.  Now, visualize getting up from your seat.  Move around your home, seeing every detail that you already know exists.  Once you are used to this, visualize a door somewhere in your house where there is no door.  Open it, go through it, and see yourself entering a completely new place that you've never been before.  Create it as you go, taking as much time as you need.  Create new rooms, or take yourself outside to a location you've never been.  For a familiar reference, use a scene in a movie or a picture in a book.  Explore the entire area, experiencing everything with all senses.
 
Once you can visualize yourself moving around in new areas, you can visualize anything.