In our web project we decided to use NoSQL, but are unsure of which NoSQL solution to use, and which languages have full support for NoSQL.
Our priorities are: availability of tutorials and support, and ease of implementation and maintenance. And we want to save the data which is from client side into both relational database (in MySql for back up and other purpose) and NoSQL db. Is it possible at the same time?
The different NoSQL solutions differ between each other much more than traditional SQL databases differ. For example there are simple eventually consistent key-value data-stores like Cassandra and other more feature-rich document-stores like MongoDB.
However, you shouldn't blindly go for the most feature rich solution, because in that case, you might as well use a relational database. SQL comes with all the features, guarantees, tutorials, documentation, ease of implementation, etc, that you can ever require. NoSQL is usually a trade-off between some of those things (or all of them), and ease of horizontal scalability.
You should research the different solutions and choose the solution that fits best your application requirements. Remember to keep relational databases in consideration, because SQL perfectly fits the priorities you listed.