This might depend on the field.
But for the most part, you want to interview as soon as they can and if they have multiple rounds (phone, meet a manager, meet the team) you want to do them as quickly as reasonably possible. They have a position open, they want to fill it, if they hire you quickly, there will be no more competition. They don't want to interview any more people than they have to, so if you're right for the job, they'll hire you fairly quickly. If they aren't willing to schedule your interview fairly quickly, they aren't particularly motivated to fill the position (or have other candidates that they consider higher priority) and might not be worth your time.
When interviewing, remember that the interview is a two way street. Try to figure out what the job really is (the hr who posted position and phone screen people likely know much less about the actual job than the interviewers). If after finding out the details, you are genuinely interested in what they do and what you would do in the position, it'll keep the tone positive, even if you struggle. An underqualified, but motivated, candidate is much more interesting than a highly qualified but apathetic candidate.
Also, by keeping the interview as a conversation (trying to figure out if/how their questions relate to what they job is), you might get glimpses of feedback on how you're doing that you will almost certainly never get directly. This should help get you an idea of if it is going to be a good place to work or not.
If possible, show usages rather than tell qualifications. Rather than saying you can do X, tell a 10 second elevator pitch style story about how you've done it before. Try to have something to say about anything on your resume. They should come out confident that anything you claimed on it was actually you, not just "your team".
If they ask about something you are weak in, admit it, and try to see how they use it and try to offer related experience that shows you understand and are willing and able to learn it. They don't really expect to find a candidate that knows everything, but if you can demonstrate that you are familiar with the space enough to learn it, they'll be more willing to give you a pass.
If you totally fail at the interview but are genuinely interested and show the ability to learn it, you still have a chance. If you show disinterest or get defensive (rather than motivated) when challenged, they won't want to hire you.
If it isn't a good fit in your mind and you'd be looking for something better the day you start, be honest with yourself and them, thank them for their time and let them know that it isn't what you thought it was. A gracious interviewee who doesn't waste their time, may get called back if they have a position that better matches your interests (perhaps even if that interviewer moves to another company). If you accept a job and leave it fairly quickly, that's a big red flag and future employers will likely check that company for references.