How to find similarity for 20 organisms on 25 parameters? How to do this ? Any software available for this?
There is not a specific software available which calculates "similarity of organisms".
Yet the tag you provided leads to right direction. You are looking for measures of dissimilarity which are synonymously called distance measures. (They are called measures of dissimilarity because a value of 0 signifies no difference.) As you did not provide information about the scales of your parameters, you can not expect any concrete advise but a general answer.
The distance measures applicability is strongly restricted to certain scales. Whereas Euclidean Distance is widely used as a distance measure of numerical vectors, LCCS (Longest Common Subsequence) is used for string similarity and DTW (Dynamic Time Warping) is used for time-series.
You can find a good overview here, but you will have to provide more information about your parameters to get any concrete advice.
For implementation of any distance measure, you can mostly calculate it (such as Euclidean Distance) in Excel or will use a statistics tool like SPSS or - most recommended - you learn programming with R or with python, where a lot of libraries offer functions that do the work for you.