There’s a rather outdated guide on this topic in the wiki: http://wiki.neo4j.org/content/Code_Contributor%27s_Guide This information will be updated and moved to the manual.

One crucial aspect of contributing is the Contributor License Agreement. In short: make sure to sign the CLA, or the Neo4j project won’t be able to accept your contribution.

Note that you can contribute to Neo4j also by contributing documentation or giving feedback on the current documentation. Basically, at all the places where you can get help, there’s also room for contributions.