With primaries more or less over, across the country, people are settling in for the long haul to November. It's been an exhausting year politically, and that's before the attack ads have really started winding up.
If you want to learn about your Democratic congressional candidate, you want to know who you're voting for and what issues you think you'll need to push them on. But in order to get there, you'll have to wade through everything on the internet. If you put their name into the search engine right now, what comes out may not be particularly useful or informative. It might be fluff from one side and possibly bile from the other, and you want something a little more honest.
So, to start, you can go to their website. How they present themselves as a candidate is important. They'll talk about what issues they want to focus on, whether or not they have any plans to push towards this issue or that. But also, in most cases, their website will also have their history. Now, it's possible you have a Democratic congressional candidate who came straight from nowhere, but that's unlikely. Often, they'll list where they were born, where they were educated (this can tell you a lot, depending on the university), what their focus in school was, and what other political jobs they might have. If they're running for US Congress, it's likely they've been in positions at the state level before — that may be where they are right now! Or they might be the incumbent for their seat.
Now, if they've held political office before, you'll want to look at their record. Govtrack.us is a great site for doing checking the voting record of US congressional members, but if they were on the state level, you'll likely have to look at that office. Think of the important bills for your state legislature that either passed or didn't pass, and look at how they voted on that. This is not to determine whether they're worth voting for, per se; it's just to give you an idea of where they sit.
You can also, of course, reach out to them through their website. Introduce yourself, and ask the questions you want them to answer. They want your input, they want your feedback, and they want to know how to invigorate their base right now, so there's no better time to reach out to your Democratic congressional candidate.