The following two practices can be implemented to prepare you for writing your first paper.
First, I do not think you are ready to write anything if you have not already read 30 ~ 50 papers in the fields you want to write about. Therefore, please consider the following Practice 1 before you even start.
Second, please try to write a lab report for a selected experiment, introducing your experiments, practice figure work, talk about analysis, talk about simulations, conclude major findings, etc. This is going to be the Practice 2.
Practice 1 - A systematic literature review:
Perform a literature review of 30-50 papers on a given topic.
Divide the paper into three groups:
Great papers: top 20% of the papers with innovative idea and solid results. You want to read them in detail - even coming back to read them multiple times as the project continues.
Decent papers: middle 50% of the papers with solid results but kind of boring to read. You may want to read the abstract, introduction, and major conclusion, summarize the key methods, go through the key figures and study them. It may take 30 minutes ~ 1 hours to read these.
Other papers: bottom 30% of papers with limited innovation and not so fun to read. May contain technical issues here and there. Usually, we just spend < 20 minutes reading them. It is not difficult to notice the lack of high-quality experiments and figures from this type of papers.
Write a report to discuss why you come up with your way of categorizing the paper. What you think are the most important aspects of great papers.
Practice 2 - Lab Report
If you work in my group, most likely there are some experiments. Thus, working on one lab report is a practice you most likely will work on - it happens before writing your first paper and also before you taking your PhD prelim exam.
Create a level-one section for your report. Every report can be different, but a general structure could be: Introduction, Experiment Design, Experiment Results, Analysis, Simulation, and Conclusion.
Prepare all the figures for your report. Keep in mind that the figure size should fit into the report format and the font size should match the figure size. See also "How to Draw a Good Figure".
Write the body of your report. Let it sit there for one day or two. Re-open the file and revise. Revise and polish for a second time. Revise and polish for a third time...
Some more tips on how to write a good abstract
How to write an abstract. I believe one of the best sources is from Nature (yes, the journal).
https://www.nature.com/documents/nature-summary-paragraph.pdf
You may also find addition discussion from Dr. Jack Baker's suggestions.