Asking for a pay rise can be a daunting task, it is often similar to negotiating your initial salary when you were offered the job but you should be in a stronger place to ask for more money. Only ask for a pay rise if you think you have a valid reason to. If you have had a lot of unauthorised time off recently, have been falling behind with work or making a lot of mistakes, then you are probably not in a position to ask for an increase in your salary.
It may be that you have structured performance review meetings and if so, this is a great time to bring up the subject of a pay increase. If not then the end of a financial year or end of a quarter may be a good time as this is when many businesses look at how they have performed during that period and if you can show that you have contributed greatly to that success, then they are more likely to agree to an increase in your pay.