Go to Slide Sorter view for your slide show. Either click on the button at the bottom right of your window...
... or go to the VIEW tab and click on the button there.
Then reduce the size of the slide thumbnails so you can see them all in your window.
Go to the TRANSITIONS tab.
Add a different type of transition to each slide using the drop-down menu.
When you click on the drop-down menu, you will get lots of options.
Now all your slides should be set to 15 sec.
Click off the slides, select just the first one and change it to "5". When you [Enter], it'll change the first one to be only 5 seconds.
Go to the SLIDE SHOW tab... and "Set Up Slide Show".
Then set it to "Browsed by individual" (this will automatically set to advance using the timings you've added).