Many people start their defence with a simple sentence along the lines of "It is denied that the Claimant is entitled to relief in the sum claimed or at all."
Following this, you set out your defence, in numbered paragraphs. You should deal with each allegation made in the particulars of claim, stating whether the allegation is admitted, denied, or you are unable to admit or deny it, but require the Claimant to prove it. From the
Civil Procedure Rules:
(1) In the defence, the defendant must deal with every allegation in the particulars of claim, stating—
(a)which of the allegations are denied;
(b)which allegations they are unable to admit or deny, but which they require the claimant to prove; and
(c)which allegations they admit.
(2) Where the defendant denies an allegation—
(a)they must state their reasons for doing so; and
(b)if they intend to put forward a different version of events from that given by the claimant, they must state their own version.
(3) If a defendant—
(a)fails to deal with an allegation; but
(b)sets out in the defence the nature of their case in relation to the issue to which that allegation is relevant,
the claimant is required to prove the allegation.
I would recommend reading as many defences on here as you can - preferably recent - and preferably not the blanket 'template' defences that state the Particulars of Claim are vague, but do not actually deal with any of the allegations.