How to get started and from where?

The key to success in Competitive Programming is- "Participate and only Participate". This is the only way you can test your knowledge and ability to think in a limited time frame.By participating you can learn to think of an algorithm in a time frame. There are various online judges that regularly conduct contest as mentioned in the previous answers.
The contests hosted by the judges are really helpful to develop confidence and also these judges have a plethora of questions to practice.
The contests hosted by each of the judge are given below.

• Codechef -

1.  Codechef Long Challenge- This is a 10 day long contest consisting of 10 algorithmic questions including one decider question that is an approximate question.
2. Codechef Cook-Off- CodeChef Cook-Off is a two and half hour coding contest where you can show off your computer programming skills.
3. Codechef LunchTime- CodeChef Lunchtime is a three hours coding contest where you can show off your computer programming skills.
• Hackerrank -

1. Hackerrank Codesprint- They are generally 2 day long algorithmic challenges consisting of 8 problems.
2. Hackerrank HourRank- This is an hour long challenge consisting of 4 challenges. Fastest coder wins.

• Codeforces -

1. Codeforces Division Contest- This is a contest that is hosted by codeforces for generally 4-5 times in a month and the duration is generally 2.5 hours.

Apart from these judges there are various other judges such as Hackerearth etc.

Some big tech Giants also host hackathons that involve algorithms like Google Code Jam , Facebook Hacker Cup and one that targets majorly college students ACM ICPC.

~~Cheers!!

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