Install Force.com IDE plug-in into your Eclipse

You can install the Force.com IDE into your existing Eclipse by following the below steps.

NOTE: Before installing the Force.com IDE, you must have to check the below requirements

A supported operating system {Windows 7, 8, or 10 / macOS 10.7, 10.8, 10.9, 10.10, or 10.11 / Ubuntu 12.04 LTS or 14.04 LTS}
Java SE Development Kit (JDK), Runtime Environment 8, or later
The Eclipse IDE for Java Developers distribution is recommended.

Open your Eclipse
Select Help –> Install New Software and Click On Add
In the Add Repository dialog box, set the name to Force.com IDE
Now use https://developer.salesforce.com/media/force-ide/eclipse45 for the location. NOTE: For Spring ’16 (Force.com IDE v36.0) and earlier Force.com IDE versions, you have to use https://developer.salesforce.com/media/force-ide/eclipse42 as the location
Click on OK button
If you want to install the Apex Debugger, select Force.com Debugger
Similarly if you want working with Lightning components, select Force.com Lightning Support
Click on Next button. Again in the Install Details dialog, click on Next button
In the Review Licenses dialog, accept the terms and click Finish
Eclipse displays a warning dialog about installing software that contains unsigned content for installing support for Lightning components. Click on Install Anyway button. And below the installation percentage shows you how much your IDE is installed on your Eclipse.
When the installation is complete, a restart dialog box occurs, click on Restart Now
After restart, select Window –> Open Perspective –> Other
Select Force.com and then click on OK button
Now you will able to see Force.com sign in your eclipse