Today's fitness tip, isn't only related to fitness.  It's a life tip, it's a company's slogan, and it has a ton of meaning to it.

Just do it.

That's all there is to everything.  Just do it.  Want to get in shape?  Just do it.  Want to land that job?  Just do it.  Want to get that girl?  Just do it.

Now, what is "it"?

"It" is that anything that you want to do.  Doing it might appear to be difficult, but it's doable.  Take a look at the end result that you want to achieve and work backwards throughout the steps that it will take to get there.  Once you get to the first step, it's easy.

Just do it.

Now, let's specify things and discuss exercise since that is my specialty.

What is your end goal?  Lose weight?  Add muscle?  Participate in an athletic event?  Win an athletic event?

Whatever it is, solidly figure it out.  Once you know what you want to do, look at the steps that need to be taken to achieve it.

Weight loss?

How much weight?  Will you have to alter your diet?  Will you have to alter your training?  Will you have to  begin training?  Do you know the answers to these questions?  Will you have to hire someone to answer these questions for you?  Can you afford to hire someone?  If not, are there other resources to be used?  Do you have athletic attire?  Will you need equipment?  A gym? 
Ok, you've gone through and answered all the questions.  You've hired your personal trainer, your nutritional counselor, and your motivational coach (or you were smart and just hired me for all three services).  You have your gym membership, your new sneakers, and for some reason a new headband.  Are you ready to go?

If you've mapped everything out already, it doesn't matter if you're ready or not.

Just do it