I'm a beginner. How do I start?

The one thing to accept before anything else: you get good at solving problems by solving problems you haven't seen, not by watching other people solve them. Everything else follows from that.

Pick a single judge and stay with it for a while, because a rating only means something once it accumulates in one place. The gentlest places to begin are AtCoder's Beginner Contests and the CSES Problem Set, both of which give you clean problems without much noise around them. LeetCode is fine to keep nearby for interview-format practice, but its easy and medium sets are heavily memorized, so lean on the others whenever you want fresh reps.

From there, work through the early roadmap topics: two pointers, sliding window, prefix sums, binary search, stacks and queues, recursion, graph traversal, and basic dynamic programming. Not because you have to clear them in a fixed order, but because they are the highest-leverage things to know and almost everything harder leans on them. Do the starred problems in each topic and follow your weak spots rather than a rigid sequence. Aim for the point where level 5 to 6 problems feel routine on the first try and under time, which is a far better target than any particular solved count.

When you get stuck, sit in it for twenty to thirty minutes before you look at anything. Then, when you finally read the editorial, take the idea, close it, and rebuild it yourself from scratch. What you're actually after is the link between the problem and the idea: why these constraints point to this technique and not another. If you can reproduce the steps but can't reconstruct that link, you've memorized a route instead of building the intuition. That whole "struggling, deriving, implementing, then coming back to it later" loop is slower than watching a video, but it's the only thing that truly transfers.