You can 1. create a folder on your Mac and sync it to your iPhone.
or 2. Create a folder in iPhone and sync to your iPhone.
I use the first option.
A. Create a New Folder on Mac - Label it. If you have multiple photos - create subfolders inside the folder.
For example - iOS Photos (street | candid | clients)
When you open your Photo App on your iPhone the (subfolders) will be placed under the last Folder on your iPhone. My last folder created on my iPhone is "Instagram"
B. Sync Photos Connect iPhone to Mac - open iTunes - Click on iphone icon - Click Photos - Select Sync button - Select folder
You would do basically the same inside iPhoto