1) Well start off by saying what course u wanna do and why. The first paragraph is always important!
2)Tell them how ur choice of AS levels help u to support ur desired degree choice
3)Talk about the work experience/work shadowing, if u've done any and if u can,try to relate it to the course ur planning to do
4)Then tell them about the school activities u've participated in and also ur interests outside of school cos the unis do not want a super intelligent person who has no life but some1 who's smart enough,sociable, willing to learn and at the same time can contribute smth non-academicly
5)There should also be a paragraph about ur results. If u did well in ur GCSEs then DO 'brag' about it cos they r v important too.If u did not do as well as u've hoped in ur ASs and are planning to re-take some modules then again tell them.
My GCESs werent so good but I did well in my AS levels so what I did is I told them why I didnt do well in my GCSEs (u can think of an excuse for urself too with ur ASs
Besides, u can always deny ur AS grades, so that unis wont know about them and will base their decision on ur GCSEs and ur predicted A level grades.
6)Finally, like in all the esseys there should be a good conclusion. Just sum up on all the good points u ve said about urself.
anyway good luck!