When i see my health bar at the last thing (where your apprentice skill or Nirnroots found are) there is written
Stores Invested In: 0

how do i make that

Stores Invested In: 1?

If your merchantile is at least 75 you get an option to pay 500g to a shop. The upside being they pay out more for your stuff (like enchated and deadric)

The downside is that by the time you reach 75 in Mercantile, you'll probably have over 1,000,000 gold and won't need to buy anything.

ye you have a good point there

In a departure from most skill level based benefits, the mercantile one only requires a current skill of 75, not a base skill.

I build a +50 for 6 seconds spell and invested in my shop of choice before I even had anything that sold for over 1200g, so you can get the benefit of the investment at a useful time. I made my 500g back plus some after doing the goblin war questline and had two 7k value staves.

-Kyrinthic

Also if you buy and sell loads of alchemy ingrediants and potions to build some gold up at the start, with mercentile as a major skill and stealth type character =P... well you get the picture I think there XD. But yeah, I think it's the number you see in your skill screen, be it blue, green or red that counts rather then you ratings in the end.