Create a diagram

Hi, I have already drawn a simple diagram which summarises an idea for a publication but want it to look a bit more professional. It needs to be saved at a high resolution for the publication and I'd need a file that I can amend if necessary (i.e. made in a widely used or free programme). I'd like all the boxes and arrows to align well, perhaps with a light grey fill for the boxes and a frame around the whole diagram. Suggestions are welcome, e.g. for adding some simple symbols.