It depends who your aiming it at? Whats the market segment? if you divide the market up i.e. is it for the 20 to 25 year old market, men or women? America or the UK! once you decide on who it is for then you can find out what there needs are, you also see where they look. so it could be best to advertise in a Broadsheet newspaper as thats the most effective means of reaching that target market! Hope this helps!
Do a search on the internet for segmentation for more details