Researching your Competition
The competition in your field of business you are moving into will not necessarily allow you to flourish as a new business and will certainly "bite back" once they see you invading their territory.
Therefore, understanding what they do and how they do it is key information you'll need to gather prior to writing your business plan. You'll then be able to understand their strengths and weaknesses and coupled with your own internal research understand how to develop better relationships with customers in the industry you are working within to grab market share and improve your profits.
Over time you should update any information you have as they may change their tactical plans on the back of what your business is doing.
Competitor's SWOT analysis
The best method is to find the competitor you believe is the best local to your business who you believe will be the toughest in your market.
Undertake a SWOT analysis on that business (see SWOT analysis for more information.)
Once you know their strengths and weaknesses, you'll have a much better understanding of how you can beat them and what their customers like about that business today.
Competition's finances
It's a good idea to understand the finances of the new company.
It will be very difficult to understand the finances for a private company as they do not have to file detailed accounts.
But you can have a look at all the information on the companies house website at companies house webcheck
Marketing and Plans
For most businesses you can see what type of marketing is being undertaken but of course there is something that you may not know about. Many competitors publish news about themselves as well as job vacancies and you can often understand a lot about a company's strategies from the types of people they want to employ and they often publish more pertinent information about their business to potential employees than they do to their customers.
Another way to find out what type of marketing activities are being undertaken is to join any mailing lists they may have (whether they be online or offline). This can uncover promotions and discounts they may be running and provide an insight in how they are treating existing customers.