I've found the best way to make money is pick up ingredients in the wild, make potions and sell them.
Easy to find
Wild ingredients are easy to find. Or you can buy ingredients from many vendors. Avoid buying expensive ingredients like void salts, fire salts, daedra hearts etc. The return isn't worth the cost.
They're light
Most ingredients weigh little. If you make a lot of potions and can't sell them all, the potions also don't weigh a lot - 100 potions weigh about the same as 1 piece of heavy armor. Their value/weight ratio is high.
They sell for a lot
I regularly sell a 0.1 weight potion for $200. There are other items with higher value/weight ratios, but I can quickly make 10 of these with ingredients that aren't hard to find. This works best for invisibility, because you can sell it for squillions. I've made around 20k mostly from potions.
I have seen potions with values in the thousands!
It's good for your alchemy skill
It'll also help your alchemy skill, and as your alchemy skill gets higher, you'll make more valuable potions.
You'll always have potions
There's nothing worse than getting into a battle without the right potions. Having a few spare potions for health and magika on hand will help you a lot!
Win win win!
Best Answer
Every time you create a potion or poison you will learn that the ingredients used have the given effects. Then when you want to create say, a potion of invisibility, you open the alchemy interface and select (on the left side) ingredients with invisibility.
This will show you all the ingredients you have on you that you know have an invisibility effect. Then you just create a potion out of them.
So no, there is no way to write down "Nirnroot + Death Bell = Poison", but you can go "I want to create a poison, what ingredients can I use". Of course, if you really want to write it down, there's always pen and paper.