Doing creative writing exercises is a great way to get over writer's block and get those "writing muscles" back into shape, so to speak. Writer's block is after all just the result of getting lazy and not writing daily, which is, by the way, something that every serious writer should make a habit of.
And doing writing exercises is a great way of achieving this goal. So even if your writing has gotten a bit rusty, rest assured that the talent is still there, it is just a little atrophied and nothing that some writing exercises can't cure.
Many books exist, which offer quality creative writing prompts and other exercises aimed at helping writers perfect the craft of writing. However these are usually focused on a specific area of fiction writing such as plot, character development, setting, descriptions and more intangible aspects of writing such as style, tone, alliteration and others.
If you are primarily looking to simply get started on the writing I would personally suggest you take advantage of some great creative writing prompts. These represent great exercises and normally involve a set of random words or phrases, which you must in some way incorporate into a story, or they can be in the form of a story starter sentence. Pictures also make great creative writing prompts.
The great thing about prompts as creative writing exercises is that they allow for a lot more creativity and offer much more free reign in the direction and form of your writing. This is because they are not as targeted at a certain aspect of creative writing as the more classic writing exercises, leaving you a lot more freedom in the direction and form your stories can take. They also allow for free writing, which is like the number one writing exercise, since it silences your critic and lets the artist come out and play.
Some examples of creative writing prompts:
Here are examples of random word creative writing prompts, random phrase writing prompts and story starter sentence writing prompts. Feel free to try you hand at writing a story from any one set of these, just make sure you use all the words or the phrases and try not to stop writing until the first draft is complete.
#1 Random words as creative writing prompts:
Yesterday, Done, Orchard, Merry-go-round, Thesis, Problem, Ecstasy, Church, Boy
#2 Random phrases as prompts:
The Oxford Encyclopaedia of Poo
Fear of Walking Slowly
A Very Good Friend Called V
Green-Top, Occasionally Red, Trying to Remember Jersey
Black Suits You, It Matches Your Teeth
The Mysteries of the Pomegranate
My Maidservant's Anne
Chocolate and the Science of Seduction
#3 Story starter sentence:
Last night the rain fell like it was never going to stop, but I was happy.
Anyway, those are just some examples of creative writing prompts as writing exercises, but if they helped you to get that first drat of a story, poem of flash fiction piece written I am glad. Now just keep at it and remember that just as physical muscles diminish if you don't exercise so does the "writing muscle."