Which sections can I skip when creating an SDS?

Most of them, actually. Out of all 16 GHS sections, only three have any required fields: Product Details (Section 1), Ingredient Details (Section 3), and Transport Information (Section 14, and only if your product is dangerous for transport).

Everything else — Hazard Identification, First Aid, Fire Fighting, Accidental Release, Handling & Storage, Exposure Controls, Physical Properties, Stability, Toxicology, Ecology, Disposal, Regulatory, and Other Information — is entirely optional. You can generate and even Confirm & Publish your SDS having left every field in those sections blank. That said, a more complete SDS is a more useful one, so it's worth filling in what you know, especially Physical Properties and First Aid.