CodeChef is probably better than TopCoder for beginners. Although there are enough practice problems of all difficulty levels on both the websites,
On a side note, you'll find this list of tutorials curated from across the web helpful regardless of whether or not you are a beginner. Also, the problems in the practice sets - easy, medium and hard - all have hint-by-hint solutions. Hints are much better than complete solutions when you get stuck because you still need to think to solve the rest of the problem, and you more intuitively understand which exact part of the solution was beyond your reach.