To build up basic skills, you have to do a lot of tactics problems. Here is the list I suggested:

• Learning Chess Step 2 Workbook (500 problems), Extra Workbook (500 problems) and Plus Workbook (500 problems)
• Learning Chess Step 3 Workbook (500 problems), Extra Workbook (500 problems) and Plus Workbook (500 problems)
• Learning Chess Step 4 Workbook (500 problems), Extra Workbook (500 problems) and Plus Workbook (500 problems)
• Chess Tactics for Champions: A step-by-step guide to using tactics and combinations the Polgar way
• A World Champion's Guide to Chess: Step-by-step instructions for winning chess the Polgar way
• Winning Chess Tactics for Juniors (534 problems)
• The Chess Tactics Workbook (720 problems)
• Chess: 5334 Problems, Combinations and Games
• 1001 Winning Chess Sacrifices and Combinations
• 1001 Brilliant Ways to Checkmate 
• Tactics Times (1000+ problems)
• Tactics Times 2 (1000+ problems)
• 303 Tricky Checkmates ​​