Hi pavelp,
I would suggest that you start filling the AOS (FORM I-864) and gather all the documentation needed (IRS transcripts, W-2, work letter, paystubs, citizenship proof, etc...) This step took me the longest. 😑
Depending on your case, gather all the civil documents (birth certificate, marriage certificate, criminal record, passport bio, photographs...) Also, make sure scanning/document files are under 2MB.
Once you have all of these documents it gets easier to fill the DS-260.
Let me know if you need anything else.