Register your business partnership
You must register your partnership for Self Assessment with HM Revenue and Customs (HMRC) if you're the 'nominated partner'. This means you're responsible for sending the partnership tax return.
The other partners need to register separately.
All partners also need to send their own tax returns as individuals - GOV.UK has further information on Self Assessment tax returns.
You must register by 5 October in your business's second tax year, or you could be charged a penalty. For example, if you started a partnership or became a partner during the 2017 to 2018 tax year, you must register before 5 October 2018.
Other ways to register
Registering for VAT
You must also register for VAT with if your VAT taxable turnover is more than £85,000. You can choose to register if it's below this, for example to reclaim VAT on business supplies.
Another option is to appoint an agent to deal with HMRC on your behalf - GOV.UK provides guidance on what you need to do to appoint an agent to deal with HMRC on your behalf.