The Money Tree grows from a Special seed, which can be found around the Sim Neighbourhoods. Money Trees produce Money Bags, which can be sold for huge amounts of simoleons!
Having a green-thumbed Sim will help you get your skills quicker, but it is not completely necessary. Remember to get the collection helper when possible: it makes it a million times easier to find seeds!
Make your sim search the neighourhood for as many seeds as possible. Pick up every single one you see, whether they are common, uncommon, rare or special.
Go to your lot and plant all of the common seeds (lettuce, apple, tomato, grape). If you collected enough, you will gain enough skill points to be able to plant the uncommon ones too (Level 5). If you haven't collected enough, simply go find some more common seeds!
Buy sprinklers and have your sim study the first Handiness book. Have them then tinker with the sprinklers until each one upgrades to self watering. This means you don't have to waste time watering your plants: the sprinklers will automatically water all plants in range every morning!
Everyday, tend to your plants. Your skill points will increase, so plant the uncommon, rare and special ones as soon as you can!
Go out daily into the neighbourhood to collect seeds. Once you have reached Level 10, you only need to collect special seeds. About 1 in every 5 special seeds will be a money tree, so collect as many as possible. Take them home, plant them and fertilize them with lobster (bought from the supermarket). It will make them grow much quicker!
Soon you will have a few money trees. Using the building option, move the trees to their orchard. Let the other plants die off, you don't need them anymore (unless you want to keep growing them).
Money Trees start at Normal standard, but over time will become nice, very nice, great, excellent, outstanding and then perfect. When they become perfect they will drop a money tree seed. You can collect this and plant it for a new tree! To speed up this process, make sure the tree is fertilized with something outstanding (perfect life fruit, for example or lobster), keep it watered and weeded!
As long as you look after your trees, they will drop seeds for you to collect and produce huge money bags for you to cash in. If you dont look after them, they will die and you will have to pay the bills they drop when they die. This is an easy way to keep your sim's family rich for generations!