You can “Hit the ground running”

I strongly believe that programmers in a new project can start productive in a day or even in a few hours. I’ve seen and experienced myself that you can hit the ground running on day one or two. This might not be true for every project but there are certain things that help getting you started.

Architecture

Wikipedia
The software architecture of a system is the set of structures needed to reason about the system, which comprise software elements, relations among them, and properties of both

A well thought out or even documented architecture in a project can go a long way. This does not need to be an all details handbook, just a rough sketch. We like to make a module map to give an overview of the parts of the system which exist and communicate which each other. But it is more important to have an architecture. Some systems get an architecture by default (see conventions) but even if they don’t you need to think and organize how the parts of your system are composed and segregated. Common rules and guidelines like low coupling, high cohesion or architectural patterns are great helpers in establishing an architecture in different levels of granularity.

Conventions

Conventions or common ways to do something in an uniform way aka style can give you a head start when diving into an unknown code base. Convention over configuration frameworks like Rails or Grails give you a set of common conventions and if you know them you can easily find the domain classes or the corresponding controller. By knowing the conventions and the style you get a rough map where to look for what.
Coding conventions help you to read and understand code (every team should have coding conventions).

Ordering your tasks

When approaching a new code base start with small tasks like changing a label in a view or fix bugs which are located in one system layer. Even better write (unit) tests to secure parts of the system you are working in or make them more testable.

Ask, ask, ask

Nothing beats the information inside the heads of the authors of the system. So if something is weird or confusing, ask. It might shed a light onto problem areas which aren’t known by the team. Nonetheless you can test your assumptions against the code (testing) or by asking your other team members.

How do you approach a new code base?