To ask for small buisness advice UK

Hi all, My friend and I are starting a new business but we haven't done anything other than buy stock and equipment. It will mainly be sales online and some event sales. We are business beginners and are employed and have PAYE tax and NI allowances. My questions are:

1. can we trade for 12 months prior to declaring the business? And tax be paid next year as it would be. As we are not yet sure how the year will go.
2. As for the expenses we have paid out so far are we taxed prior to expenses being removed?
3. With regards to a bank account with my friend would we set up a personal account between us or a buisness account. Or is there another way?

I have been talking to some and they have really negative views and it makes it feel like red tape after red tape to get through. But surely it isn't too bad because lots of people are self employed.

Please can you give me your best buisness tip for total beginners who want to do a good job of it and be profitable.

Thank you in advanced.