I have heard lot of issues from the people who are new to OpenMRS development. So thought to write my own guide. Most convenient way for me is using the Maven command line. No need of IDEs you can build and Run OpenMRS without any hassle.

These instructions will work within any platform regardless of the OS, You may need to configure the path variables first of all to use them on terminal. (How to Set Env. variables for Maven?)

Here are the simple steps.

Prerequisites: Apache Maven, JDK, MySQL server

1. First Clone the Git Repo: https://github.com/openmrs/openmrs-core

2. Navigate to the /<path>/openmrs-core/ directory using command line.
Fire > "mvn clean install"

You will need internet connection at this point. Cause maven will download all the required dependency jars from servers. First build will take a quite a lot of time depending on your Internet connection. In second time and so forth It will build really fast within few seconds. 

So dont worry about itAfter the first build to speed up the build process, you can use "mvn clean install -DskipTests"  command to skip the junit test cases. First time you need to build with test cases so you need to use the "mvn clean install" command.

3. Navigate to the /<path>/openmrs-core/webapp/ directory using command line.
Fire > "mvn jetty:run"

4. Now browse "localhost:8080/openmrs/" and you are done.

Really simple :-)

If you have come up this far you have already built the code and Ran OpenMRS system by yourself in your PC, Satisfied coder and ready to play with the code base. :-)

So IMHO After trying these steps try to import the project to Eclipse/IntelliJ and see.
Sometimes Eclipse gives crazy errors, (crashes, proxy, and lot etc) So OpenMRS starters tend to get demotivated by facing those little issues. We can blame Eclipse for it, Which is not a fault of OpenMRS code base :-D

Further Reading: https://wiki.openmrs.org/display/docs/Getting+Started+as+a+Developer

Hope this helps!