Depends what's wrong. If you only need a vision check, for possible glasses, then go to an optometrist, but if you have a problem, other than a vision problem, with your eyes, then you may need to see an ophthamologist, who is a doctor who specialises in eyes. But you'll need a referral from your GP first. So I'd go and see either an optometrist or GP first; they will let you know whether or not you should see the specialist.