Rob has answered this perfectly. However, I would add, don't try to spread yourself too thinly. In my experience (by no means extensive btw) I've found PPC to be a bit of a numbers game - you need quite a lot of clicks before you get a conversion.
I suggest you try a remarketing campaign if you only have a low budget as the clicks are often cheaper, and the traffic highly targeted (and has been exposed to your brand previously, so more likely to convert).
When I set up our remarketing lists/campaigns I found this article incredibly helpful: http://www.ppchero.com/ultimate-guide-to-adwords-remarketing/
I've been running remarketing campaigns for several months, and whilst the ctr is considerably lower than search campaigns, the conversion rate is as high or higher, and of course the cost per click is minuscule in comparison. I find it a cost effective solution and helps convert traffic that may never have revisited the website otherwise.
Depending on how much traffic your website gets, you may have to wait a while for the lists to populate, so I'd set some up anyway, as you may want to switch it on during a busy period (such as mothers day if you sell flowers, or christmas if you sell gifts - whatever works for your business) - better to have something up your sleeve and never use it than want to use it and have to wait!
Good luck and best wishes,
Amelia