https://www.hackerrank.com/ and https://www.hackerearth.com provides good track guides to begin with.

https://www.codeforces.com, https://www.codechef.com/, https://www.topcoder.com/ (even if in my opinion, this last one is becoming worse and worse) provides good problems and contests to apply and discover new ideas.

And if you're even more motivated, upsolving previous contests problems. Codechef long contests are really interesting for this: after thinking a lot about these problems during more than 1 week, reading the editorial and trying to apply by yourself the idea seems to me an excellent way to learn.