If you don't need to modify the sources, you can just download the jar file from here and import it as an external library.
If you need to modify the sources, follow these steps:
To get started, you need to install the Maven-Plugin for Eclipse. Click on Help > Eclipse Marketplace. After it finished loading, search for “Maven”. Now install “Maven (Java EE) Integration for Eclipse WTP”. You have to restart, after the installation is completed.
Now you need to get the project itself. Visit TelegramBots - github.com and choose “Download Zip”, to download the zip-file. Here is a direct link if you are lazy.
Now let’s setup the project-folder. Go to your Eclipse-Workspace and create a folder (name it whatever you want it to be called). I just named it “TelegramBotApi”. Afterwards copy everything from the zip-file into that folder.
To import your project into the workspace you have to go to File > Import > (Folder) Maven > Existing Maven Projects > Next. Choose the folder, you have created in Step 3. In my case: “TelegramBotApi”. Click on finish and let Maven import the dependencies.
In this last step you need to change the Compiler compliance level. To do this right-click on your Project (in my case “TelegramBotApi”) > Properties > Java Compiler. Uncheck the “Use compliance from…” if necessary and set it to 1.8
Now you are done. Everything should work fine.