Depnds what the ad is for. Where will it go, what does it have to compete with, what product or service are you advertising etc. Isolate all the elements that you HAVE to have (eg company name, logo), then sort out the bits that are discretionary (Which have to be there but can be changed, eg the copy or the message you are trying to communicate), followed by the bits that are optional (ie the bits you might like to include but could possible do without).
This
book gives a whole load of layout possibilities. Start with the must-haves, then the discretionary bits, and add any optional bits only if they add to the whole design. Less is more (usually).
Decide on a colour pallette early on (which will be dictated to a large extent by the corporate colour scheme and logo). Use an on-line colour selection site. Colour will also affect layout - strong colours pop out of the page and light pastel colours recede into the background.
If you are working for a company that has a really strong brand image you can really go minimal (eg a Macdonalds ad could just consist of the golden buttocks logo and the word 'Mmmmmmm...') and get the message across, whereas an ad for the local bike repair shop would necessarily have to impart much more information to be effective.
But there's much more to it than this, which is why graphic designers do a 3 year degree course.....
