Most barristers begin their career being instructed for the defence, and it is likely that in your second six months of pupillage (and probably throughout the early part of your tenancy) this will constitute the overwhelming majority of your case-work. A defendant has every right to maintain their innocence until proven guilty. Defendants contest cases because they instruct their solicitors and barristers that they are not guilty, and have been advised that they have a defence to the charge. The barrister will challenge the evidence called by the prosecution, cross examine witnesses with a view to undermining the credibility and reliability of the witness, and make closing speeches trying to persuade the judge or jury that the Prosecution have not proved their case.
Some barristers even become additionally qualified to conduct Direct Access work, so that potential clients can come straight to them rather than needing to go via solicitors.