Filtering Problems

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There are a plethora of problems available on various online judges and it can be overwhelming at first to find problems suitable for your level. Here are some of the available tools which can help in classifying problems based on topic, difficulty etc.

SPOJ (Sphere Online Judge)

http://problemclassifier.appspot.com/

SPOJ has a large collection of problem and is usually the first place most people start competitive coding from. Since the problems could be added by anyone there are many repetitive problems on SPOJ and it might be difficult to find good problems. Using the above website one can filter problem based on tags like "dp", "greedy" etc and also difficulty "medium", "hard".
The other thing is to sort problem based of number of users solved (Which is roughly inversely proportional to difficulty) and solve the initial problems. The site also mentions two type of difficulty which are "Implementation Difficulty" and "Theoretical Difficulty".

A2OJ (A2 Online Judge)

https://a2oj.com/ladders

This works on ladder based system in which users are requested to join a particular bracket of rating (usually codeforces) and there are a set of problems given for that rating range. You are free to do topic wise problems and advanced filtering.

CF-Task-Filter

https://github.com/Timus1712/cf-tasks-filter

This is a scrapper which filters problem on codeforces based on codeforce difficulty system like Div 1 A , Div 2 C etc. You just need to clone this repository and the run 

python filter.py

The program will prompt for your handle and after that it will provide you with a webpage of problems classified into difficulty level. This is a very useful tool for codeforce problems.

Apart from these most other online judges have their own difficulty level like "Easy, Medium, Hard, Challenge" for codechef and "Easy, Medium, Hard, Advanced, Expert" for hackerrank which give a good idea. Hackerrank also has topic wise problems like "stacks, queues, graphs" in the practice mode.