Should a pupil volunteer more sensitive material, this can be a stimulus for some wonderful writing, but always be aware of safeguarding issues.
Contemplating an autobiography can be daunting and, as with any extended writing task, difficult to find a starting point. As a solution, why not try a good old-fashioned spider diagram or mind map. Get as much down as possible, but ensure this is a time limited task to ensure focus.
Pupils could do a spider diagram fro each year of their lives, highlighting trips, holidays or the arrival of siblings and pets. This starter exercise should yield so much information to write about, they might want to whittle it down to a few key events - possibly three or four to focus on. This will enable more detailed paragraphs and more variety of sentences.
Sensory writing should be encouraged when thinking about memories (how did it look/sound/taste/smell/feel), as well as some other figurative techniques (you can download a set of posters here to help with that).
It will be important to recognise that this will be a first person narrative and past tense will feature heavily - you can check my article regarding some common mistakes with past tense here.
Finally, it will be important to revise the use of adverbs of time. These can prove invaluable with autobiographical writing for connecting ideas, so make sure you have a bank of them handy:
firstly, then, before, usually, finally
The bonus of using adverbs of time to connect the ideas, is that you will also encourage the use of fronted adverbials in sentences, which will make the whole piece of writing more interesting to read.
You can download this PowerPoint to use in class and highlight the main ideas.