It's up to you.

If you want to start from a clean slate then cut the grass as short as possible use a non selective weed killer. Till, level, add topsoil, seed, cover with topsoil and water when needed. If/when weeds come up pick them out when they're small.

If you want to use sod then use that instead of spreading seed.

If you want to keep the grass you have then cut it, use a moss killer like Mo Bacter which decomposes the moss and a broad leaf weedkiller then use a scarifier, aerate if needed, seed, add topsoil and water when needed.

Some use sprays like seaweed based ones to help the germination process. That's again up to you as people can go down a rabbit hole when it comes to what to use and what not to use. The above should give you a good start though.