Conversion From Square To Sine Wave...(Filter Design)

Hi,
I need to if there is any simple methode to convert a square wave to sine wave of a modified sine wave to pure sine wave for an inverter.
i dont have prior knowledge of filters as i searched on google and there are some LC type filters i am familiar with Inductors and capacitors but never used as filters before.
i have also heard that transformer at step up stage also acts as filter and gives sine wave plz help in this regard.
Thanks In Advance.

Depends upon how much distortion you can tolerate in the sine-wave.

What is the form of the "modified sine wave"?

I need pure sine wave no discrepancies....
ok i have a c code for spwm and its working fine and at output stage i am using a low pass filter and results are very good i am getting a good sine wave but the problem is i am using RC filter but in actual i dont know what values of RC should be used i mean their power ratings and i have seen some LC filters but i am struck in designing one as it will be working on 230VAC so i am little confused about it.
thanks

One More thing i am interested in designing SMPS inverter to reduce size of transformer i want to know like usual 50 or 60 Hz transformer..ferrite core can be used in reverse? what if i get one from old laptop power supply and use it in reverse to get 310vdc?