What should be the scope and the contents?
A thesis is a self-contained document. Every physicist should be able to understand the contents without additional literature. Every none physicist should get an idea of your work. Give a reasonable introduction to ensure that everybody gets the required basics to understand your work. You can concentrate on the aspects which are important for your topic, but make sure to give a proper citation so that the interested reader can get additional background information. Do not repeat the contants of existing previous works or papers. Summarize them reasonably in the view of your thesis and properly reference them.
Where do I need citations?
Make sure that all statements in your thesis which are not standard textbook physics or a statement of your own work are properly cited. If you copy text from somewhere, because it is better than any text you could produce and fits perfectly, ensure that it is marked as quotation. Make sure that you properly reference your graphics if they are taken from somewhere else and indicate if you modified them. A PhD thesis is a publication! To publish copyright protected material in your work permission of the right owner is required. It is debatable how much modification is required to make it 'your own work'.
How should my bibliography look like?
The bibliography is equally important as the text. Ensure proper formatting. Reference must be complete (just copy&paste from some BibTeX entry you found somehere is often enough). It must be easy to find a reference, that usually means it must contain at least: Who, where, when and ideally also what. References with 300 Authors is not a good style. Use one or three authors 'et al.' (consistently). If possible add the DOI (ideally as link), or if not available the arXiv number, e.g. arXiv:1304.1710. For web-links, provide the permalink (if available). 'Private Communication' is only acceptable if there is really no other source for this information. If this is a collaboration paper, it is a good style to add the collaboration, e.g.
Can I use abbreviations?
Abbreviatiosn are usually not a good style. Try to avoid abbreviations and make sure they are always properly introduced (ideall in each section individually).
How to format my plots?
Make sure that all axis are properly labeled. All labels in the plot should have a reasonable size (as a rule of thumb they should not be smaller than the surrounding text). If your plots are a result of different coefficients, cuts, or other changing variables, ideally they are found in the title of the plot but at least must be part of the caption. Make sure that your legend is not even close to your data points, in particular they must not cover your data points (not even the error bars). Ideally, put the legend outside of your plot.
Can I use footnotes?
In almost all cases, footnotes should just be part of your text. Just format your text accordingly. Only statements or technical details which really disturb the flow of the text should go to a footnote. Generally, they should be avoided.
Which language to use?
You are free to writ ein German or English. As the language o physics is English, it is recommended (also for yourself) to write your thesis in English. This is not a language contest. With the help of your colleagues, the language in your thesis will be good enough.
Some simpel language tips (English)
No comma before that
Better use a gerund than which, e.g. 'The current which was flowing through the wire...' -> 'The current flowing through the wire...'
In contradiction to German, everything which can be read as a sentence on its own should be surrounded by commas, e.g: 'Although, the weather is good today, there might be some rain.'
Active is always preferrable over passive
Avoid using active in the form 'I did', 'We did'. In a thesis where 'I did' might be correct, in almost all other context these phrases are wrong (who is 'I' or 'we' in a 30 author paper?)