Hi -- your question is quite general. Have you read the User Manual? It's my first resource for Aperture. Here is the chapter on keywords:
Your general workflow will be to group Images to which you want to apply the same keyword in a selection, and then apply the keyword. There are many ways to apply the keyword to a selection. I use either keywords assigned to Button Sets on the the Control Bar, or the Add Keyword field, which features auto-complete.
Others disagree, but I also suggest (unless you are a professional stock photographer) deleting the entire list of keywords that Aperture ships with, and creating your own, completely-customized-for-your-use, list.
If you do use the Add Keyword field, you might also use a strategy of using only capitalized keywords, but entering them as un-capitalized. Auto-complete will capitalize them -- and an un-capitalized keyword in your Keyword HUD indicates that it was mis-entered.
There are lots more hints and posts regarding keywording in the forum. A search here is likely fruitful.
You're welcome. I linked to the on-line User Manual. This is also available locally (stored on your computer). As with all programs, you can access it via the Help menu ("Help➞Aperture Help").
The Apple Aperture Support page is another good resource to know about.