Hi
If you are self-employed, you need to register for NIC (3 months limit and a penalty failing that). Also, need to notify the HMRC for income tax by 5 October following the end of tax year in which you start trading.
If you are a limited company, similar deadlines with penalties to notify HMRC of trading commencement.
Need to register for VAT when turnover roughly within a month after reaching threshold or straight away if bought an already registered business or if reasons to believe that threshold will be exceeded within a month (e.g. large order).
Also, need to think about registering for self-assessment as a director, register as employer with HMRC as an employer if you employ yourself or others.
Also....
A competent accountant can come in handy if you are not sure what to do-their fess can be more than offset by helping to avoid non-compliance penalties let alone organise your affairs so as to save tax.