Hi,
Form SA1 is where you are not registering as self employed. As you intend becoming self employed you register for Self Assessment differently, so that national insurance is also notified.
This is done by following the guidance at:
Set up as self-employed (a 'sole trader'): step by step
If you have not commenced self employment yet, but know when you are likely to start, you should use the date you belief you will commence trading.
If you are unable to register online, you can complete the online form CWF1 below,
HMRC: CWF1
You can print it off and post to:
Self Assessment HM Revenue and Customs
BX9 1AN
United Kingdom
Please note that sending the completed form by post will take longer to process.
Thank you.