wilkinson power divider in ie3d
Stripline impedance is a function of line width, separation, and dielectric constant of the separator.
Simon and Waddel are useful references and there are numerous applets on the the web to get you started. Polar Si9000 is also a good tool but there is a cost associated with using it.
You might also like Microwaves101.com - A practical resource covering the fundamental principles of microwave design (w*w.microwaves101.com) as a source for information. If you have access to a old divider you might consider opening it to see how it was fabricated and the study the internal layout.
Since you have a EM tool you might consider modeling a family of striplines to see how they behave to start. Use the materials and dimensions you intend for your design. Then look into the wilkinson.
Azulykit's advise is excellent!
cant-bury,
There are a lot of books explaining impedance calculations in microstrips. If you want detailed info. have look check for Microwave Engineering, by David Pozar.
Just draw a structure of a wilkinson and play around by changing different parameters. You'll get hands on experience on how it is working.
AWR makes a free app called "TxLine" which is an impedance calculator for a variety of transmission line types...i use it frequently and it gives very good results. My only caution is it is analytically based so a 3D solver is sometimes useful for sanity checking (I have found strange results, for example, with the coupled line model in TxLine). AWR | TX-Line | Optional Products - TX-Line