The requirements depend on the route, but a family visa application may need evidence of your identity and nationality, your relationship to the person in the UK, the UK status of your sponsor or family member, your financial position, suitable accommodation, English language ability where required, previous immigration applications, any previous refusals or overstaying, suitability issues, and where relevant the circumstances of any children.
A spouse visa, parent visa, child visa and adult dependent relative application can all require different evidence. A solicitor can help check whether the evidence matches the correct route, rather than simply preparing a generic document list.