The Government have posted some guidance to help you get prepared for Brexit. You can find the details by following the links below.
Preparing for Brexit https://www.gov.uk/brexit
Get your business ready to export https://www.gov.uk/prepare-export-from-uk-after-brexit
Get your business ready to import https://www.gov.uk/prepare-import-to-uk-after-brexit
Transitional simplified procedures (TSP) https://www.gov.uk/guidance/transitional-simplified-procedures
Getting help from third parties https://www.gov.uk/guidance/appoint-someone-to-deal-with-customs-on-your-behalf
Deciding which customs procedures to use https://www.gov.uk/guidance/customs-procedures-if-the-uk-leaves-the-eu-without-a-deal
Common Transit Convention (CTC) https://www.gov.uk/guidance/what-you-need-to-move-goods-between-or-through-common-transit-countries-including-the-eu
National Export System (NES) https://www.gov.uk/guidance/export-declarations-and-the-national-export-system-export-procedures
Grants for businesses that complete customs declarations https://www.gov.uk/guidance/grants-for-businesses-that-complete-customs-declarations
Help and support for traders when the UK leaves the EU https://www.gov.uk/guidance/help-and-support-for-traders-if-the-uk-leaves-the-eu-with-no-deal
Prepare your business for Brexit (useful tool) https://www.gov.uk/business-uk-leaving-eu
HMRC Customer Forum https://online.hmrc.gov.uk/webchatprod/community/forums/list.page
EORI mythbuster https://www.gov.uk/government/publications/hmrc-brexit-communications-resources