Which software are you using to design the PCB? This would be helpful to know to provide you with more specific answers.
My favorite calculators, which I have found to be most accurate, are those at eeweb.com. Here is the single-ended microstrip calculator, though other calculators are also linked to the right on the following page:
**broken link removed**
Considering you have a microstrip (a trace over a ground plane) then you'll want the microstrip impedance calculator. If you have differential signals you'll want the edge-coupled microstrip calculator. More details regarding what you're designing (perhaps a screenshot) would be very helpful.
Again, more details are required. Trace length is important to match propagation delay of different signals. Some receivers will not behave properly if, for example, the clock signal reaches it before the data signal does. In such a case, suppose the data contains a low-to-high transition and clock tells the receiver to pass the data through before the data is received, instead of clocking through the new high-level signal it clocks in the old low-level signal. Your data which is transmitted will have incorrect bits. It is important to ensure that the signals reach the receivers at the desired time to ensure they are captured properly. Matching trace length isn't as important as matching propagation delay, so that is what you'll need to calculate. Tools such as this one will help you calculate the propagation delay based on your trace dimensions:
As kubeek suggested, I too recommend the
Saturn PCB Toolkit. It's free and has a lot of very useful tools for PCB design.
Good luck!
Matt