If you're completely new to programming, your first tasks will probably involve learning to code. (You'd be hired to an internship or "learn-on-the-job" position and be expected to be a student for the first few months.)
If you're fairly new, it depends on the company. Some companies would just have you pair program with one of the existing developers. You sit next to them for a bit, while they do their normal task and explain to you how things work. At some point during the day, you switch so that you're working on a regular task and they observe and guide you. You do that for a few days, and then you can work on your own. (Or keep pairing; some companies pair-develop everything)
Other companies have you read up on the code base first, and give you a few days to look around and get familiar with it. You can fix some minor bugs, write some test cases, or implement some trivial features until you get an idea of what you're doing.
And probably there's a dozen other things companies do, although for some I question the efficiency.