The Statement of Truth is very important. Also make sure that it is headed up as a Defence to make it absolutely clear.
As the others have said, keep it to the point and numbered paragraphs are good, also keep things ordered chronological etc..., attach evidence if appropriate such as invoices/receipts. Send copies rather than the originals and keep copies of everything that you send.
The easiest way is to use the Defence form provided in the Response Pack which should have been attached to the claim form when you received it.
Is it a claim for a specified amount of money or unspecified amount (ie to be decided)?
The forms are different but you can find them on the Court Form Finder - a Defence form is form N9 but make sure you get the right one if downloading a copy (as said, easier to use the one in the response pack if possible but if you want to type it then make sure it's the correct form). It's N9B for specified and N9D unspecified:
http://hmctsformfinder.justice.gov.uk/HMCTS/FormFinder.do
You can also download a guide from the same search, type in EX303 which gives some information about defending a claim generally.
You will see the Statement of Truth referred to on these forms too.
Note that a copy of the Defence will be sent to the claimant.
If you are making any counterclaim, make sure you send the appropriate counterclaim fee with it.
Finally, make sure you are aware of all the timescales by which documentation needs to be filed so you do not risk having a judgment entered against you.