Dear,

For your preparation being from a commerce background can help you as it forms a base as the syllabus is quite similar but not while selection. Incase you think the commerce students have an upper hand in the selection process then this is not true.
Incase you have a work experience of 2 years and above then it shall be considered. Also it shall be seen how much relevant it is and what have you leant from it or what skills you have gained from it.
Work experience is not mandatory. If a student is a fresher then also he/she is eligible for admissions in IIMs and keeps good chances if the percentile and other parameters are good than  the one having experience.
To get into IIMs main considerations are:
1. Past academic scores
2. CAT percentile
3. Work experience
4. Other selection round

Thanks