First thing you'll need to do from a tax perspective is put a current market value on the item - you can't introduce it at the price originally paid. As it's a computer, digging around the 'recently sold' section of ebay will probably be of some good use. Be sure to save down copies of your findings so you can provide backup to your valuation should HMRC ever ask.
With regards to the bookkeeping entries within Clear Books, there are a couple of different methods you could use but the easiest will probably be a quick journal. You'll need to first make sure you've got a Directors' Loan Account setup by going to Money > Bank Accounts > Add account > Directors' Loan Account. This account will keep track of all monies owed to you which you've loan/invested into the business (such as introducing a computer). You'll then need to create the Journal itself:
Tools > Journals > Create
Enter a brief description of the asset, date it as of your first day of trading, leave the number of journal lines as 2 then enter the following:
Account (Plant & Equip - under non current assets) £X debit (with X being the value you've placed upon the item)
Account (Directors' Loan Account) £X credit (again with X being the value you've placed upon the item)
Finally, if you're keeping track of assets (which i'd advise you to do), you'll need to head to Tools > Fixed Assets to firm up the details.
Hope that isn't too confusing, feel free to shout if you need me to re-run through anything in more detail.
Kevin @ Platform Accounting