Well it depends on each position. Dont go by the games' evaluation of best postion.
Check out important skills like for a point guard,outside defense, passing and jump shot are important. secondary skills for a pg are handling,driving and jump range.
Please go through the links and undersatnd the training concepts. Also you can choose to train young players of less than 20 years and in a few seasons they can gain good skills.
http://buzzerbeater.wgz.cz/rubriky/main-skills
http://www.buzzerbeater.com/community/forum/read.aspx?thread=144856&m=23
The above link is really important for any new player and i suggest you go through it.
Anything else you can ask the managers here.
Hope i helped :)