A proposal is similar to a report. The main difference is that in a report you refer to a current situation whereas in a proposal you will have to concentrate on future recommendations to improve the present situation of the issue you are dealing with.
The structure is usually as follows: introduction, current situation, suggestions and final recommendations.
You should write in a formal style.
Do not use abbreviations or any kind of slang words!
Try to use passive grammar sentences. So, instead of "I think that the school should..." it is better to write "It is recommended that the school..."
INTRODUCTION
The aims of this proposal are to...
This proposal evaluates....
This proposal is intended to present the...
CURRENT SITUATION
Many students/local residents/pensioners/members of the local community have commented that...
There is a strong feeling among all concerned that...
Following a survey of local residents it was found that...
SUGGESTIONS
It is recommended that...
There should be...
It would be advantageous to...
FINAL RECOMMENDATIONS
If the above suggestions are implemented there would be...
Unless the recommendations are taken seriously, it is unlikely that...
Implementation of the above ideas would result in...
(From http://www.stgeorges.co.uk/blog/academic-english/cae-proposal-example-sentences-and-structure)