There is NO particular IDE for creating JSP files only, I know Eclipse Europa, Netbeans has a very good support for JSPs EL, JSTL, etc functionality. You just need to read a tutorial or two to get stated with them.
And if you're more concerned about Look and Feel, then go for Dremviewer or any HTMl builder software to design HTML code which can be incorporated later into JSPs.
If you are using Eclipse 3.1, please follow the below steps to use your exisiting code to create a project.
Step1: File -> New -> Project.
You will get a dialog box to choose your project type.
Step2: After seleting the corresponding project type, click on 'Next'
Step3: In the 'Create a new project' dialog box, click on the 'Create project from existing source 'radio button.
Step4: In the Directory textbox, give your path of source code of existing application.
Step5: Read the other options and select appropriately whichever you need. Otherwise click on 'Finish' button.