Given the large amount of administration required in running your company, you will need to find a good accountant to manage this work on your behalf.
Typical services performed by an accountant would include some or all of the following - make sure you check what is included in your monthly fee before signing up:
Payroll administration
Processing of PAYE / NI payments to the Inland Revenue
Preparation of Company Year End Accounts
Personal Taxation - completing your self assessment forms
Completion of VAT returns
Completion of P11D, P35, P60 and other forms on your behalf
Tax Planning Advice (e.g. Dividend Planning)
Typically, you will expect to pay £60 - £90 per month for a reputable firm. Make sure you find out what this fee includes - beware of hidden 'extras' (such as a £15 monthly charge simply for having a company car).
Many contractors have tried several accountants over the years, before settling for one they feel comfortable with. At some stage, you may feel the need to move accountants for whatever reason - the best time to do this is after a company year end, since most outstanding paperwork should have been completed by this stage.
So, ask other contractors for advice, or take a look at the accountants advertising throughout CUK.
If you have decided to trade through an Umbrella Company you would only need an accountant to prepare your Personal Tax Return at year end, and your umbrella Company should be able to recommend one to you.