there are a lot of tools and libs with the functionality available.
Which to choose depends whether you would like to use a gui for your work or if you would like to embed it in some other program.
Standalone Data mining tools (there are ohters like WEKA with Java interface):
- Rapid Miner
- Rattle gui for R
- Scikit for Python
- Mahout on Hadoop
If you know a programming language well enough I would use a lib for that language or give R a try. If not you may try one of the tools with gui.
A tree example in R:
# we are using the iris dataset
# for our tree based model we use the rpart package
# to download it type install.packages("rpart")
# Building the tree
fit <- rpart(Species ~ Petal.Length + Petal.Width, method="class", data=iris)
# Plot the tree
As suggested the analysis with R requires you to code yourself, but you will find a package for most classification tasks which will work out of the box. An overview can be found here Machine Learning Task View
To get started with RapidMinder you should have a look at Youtube. There are some screencasts, even for decision trees.