When you purchase a candle you have no idea what energies had previously been attained. It would be best to cleanse and purify the candle before empowering it with your energies. I suggest these simple forms of cleansing:

-take a lit tealight and hold the candle to be used above it(not too close) along the outside of the flames glow

-rub the candle with a soft cloth with a little rubbing alcohol

Directing the candles purpose is very important. Some prefer to do this by inscribing symbols, pictures, letters, runes, etc. The most popular is the Witches' Alphabet because the time consuming effort adds greatly to the charging of the candle itself. When inscribing always start from the wick down to the base(never side-to-side). You want to go with the flow of the candle not against it.

Next you will need to dress the candle by annointing it with oil(usually essential oil). Some people find it more effective to dress it with homemade essential oil. To annoint the candle start from the middle and rub the oil on to the ends until covered. Visualize the need and you're candle will be fully prepared for the spell. For best results, do the spell at night with all the lights off. Only the light of the candle and the light of the moon should be present.