To submit a tax return online, that includes the residence section, you will need to purchase third party commercial software. You can find a list of suppliers of this software here:
Self Assessment commercial software suppliers
To determine if split year treatment applies to you, you would need to read the guidance here:
Guidance note for residence, domicile and the remittance basis: RDR1
You will then need to apply the statutory residence tests here:
RDR3: Statutory Residence Test (SRT) notes
If split year treatment applies, then this should be declared on a Self Assessment tax return, either in paper format or using the third party software to submit the return electronically.