What potion can I easily create?

One of the easiest and most useful potions to make is Restore Health. You can make these by the truckload with ingredients around Whiterun. Look for:

• Wheat - which you can find as large bushels in the farms surrounding Whiterun, as well as some other farm locations in the plains west of Whiterun. These bushels respawn, eventually, although it takes a couple of weeks.
• Blue Mountain Flower - which is in abundant supply in the non-farm fields around Whiterun, and fairly common across the plains of Skyrim.
• Butterfly Wing - these can also be found by catching the reddish/orange butterflies that flit about in the fields around Whiterun.

Mix these three things together, and you'll have a Restore Health potion. Don't forget to check the alchemy shops in Whiterun and other major towns, as there's plenty of these low-level ingredients on offer, and once you have some alchemy ranks you can easily profit by crafting them into potions. When shopping, look for Blisterwort as well, although be careful of side effects from various ingredient mixes - which ones are dangerous is left as an exercise to the reader. :)

Now, for profit purposes, you'll want to throw a Giant's Toe into the mix. Giant's Toes boost the potion's profitability by a crazy amount. There's a list of locations in that linked article, or you can just kill giants, if you've got the levels for it.

Put those previous three ingredients together with the Toe, and you'll end up with a worthless potion/poison that has tremendous resale value.

Not counting special ingredients like Giant's Toe which are difficult to get in quantity, you can mix Blue Butterfly Wing and Blue Mountain Flower.

There are a couple butterfly spawns by Honningbrew Meadery and the nearby crossroads/bridges. These particular spawns seem to reset when you enter and exit the meadery.

For the flowers, head north past the farms and Whitewatch Tower until you hit the road heading east and west between Dawnstar and Windhelm. As you approach this area you should find some.

For more ingredients, keep heading east and north(possibly go by Fort Fellhammer instead of the other fort for less bandits to deal with) and when you hit the fork in the road approaching Dawnstar, head west again. Go by dwemer ruins and Frostmere Crypt until you get to the Stone Hills mining community(massive butterfly spawn here, too). You should have encountered tons of blue mountain flower along the way.

This combo makes poisons of damage magicka regen, the 2nd most valuable alchemy effect after paralysis.