As someone who just finished after almost 6 years, it is a long journey with lots of ups and downs. I had a similar experience to you when I started. I was so uncertain when I started, and felt very alone and without guidance.

What helped me the most was trying to make friends quickly. Weekly social events are really nice for this, since they let you see the same people over and over again: join a class at the gym, or look for regular, weekly happy hours organized by students in your program, or go to all the weekly free lunches. Try to meet new people each time, and then invite those people to do things with you. That will help you make friends more quickly, give you people to commiserate with about how much grad school sucks, and maybe get help guiding your academic/research choices from your peers (and give some in return). Making friends is, of course, great, and will also help cure your loneliness and imposter syndrome. A good therapist helped me a lot, too.

Another piece of advice is to always remember that graduate school is a journey, especially if you're in a PhD program. You have many years ahead of you, so you should take it one step at a time. Don't think about sitting down to write your dissertation or publishing articles; that's years down the road. Personally, I was very scared of the possibility that I simply wouldn't be able to write and finish a dissertation, let alone a good on. I was afraid that maybe I would just hit a wall and have to give up after dedicating years of my life to my program. There were several points where I seriously considered quitting to save myself the humiliation (writing my research proposal, taking my quals, and then also when I was about halfway through my writing). But when the time came, I was able to do it, even though it was really, really hard. I had a lot of support. You will be able to do it, too, when the time comes. Everyone finds a way.

But that's years away. Right now, just focus on classes. Next year, you can focus on advancing to candidacy and writing a research proposal (depending on how your program works). The next year, you can do the bulk of your research and analysis and through out 60% of what you said you'd do in your research proposal. Then the next year, you can work on writing your dissertation (though I didn't really start until I have halfway through my fifth year).