Hi! In my experience, it’s often clear who is the main author, who is then put first. Sometimes the second author has done an equal amount of work, and I consider it good practice to make that known. Additionally, I like to add a brief statement about the authors’ contributions. There are many guidelines to write such a statement. For example, this is the one of the Royal Society Open Science:
All submissions, other than those with a single author, will require an Authors’ Contributions section which individually lists the specific contribution of each author. The list of authors should meet the criteria provided on our policy page. All contributors who do not meet all of these criteria should be included in the acknowledgements section.
We suggest the following format:
AB carried out the molecular lab work, participated in data analysis, carried out sequence alignments, participated in the design of the study and drafted the manuscript; CD carried out the statistical analyses and critically revised the manuscript; EF collected field data and critically revised the manuscript; GH conceived of the study, designed the study, coordinated the study and helped draft the manuscript. All authors gave final approval for publication and agree to be held accountable for the work performed therein.