Upload your SVG file to begin. A temporary (you have been warned!) PNG rendering of your SVG will be generated, at 1:1 size, and exactly how the Wikimedia servers would render it, warts and all.
Additionally, if you have ticked the 'Check this file for errors' box, then an output showing any automatically detected (possible) errors will be generated. This is designed for files known to be causing errors; it is therefore inclusive and will give more output than is normally necessary in order to ensure that the problem is found. Some errors are fixable using a GUI like Inkscape; others will require you to edit the file using a text editor like Wordpad.
Please be patient, the process can take some time. Some filenames represent a possible security risk and will be blocked. Queries can be directed to me on-wiki or to my username at Gmail (it stands for Google email, you know) dot com.