Want to improve your chess? Start here! I have watched and analysed and commented on many junior chess games over the years, and I am pretty clear now about the most common reasons for losing a game [or drawing a game you were winning]. They are:
* missing basic tactics
* not sure what to do in the opening
* not sure what to do *after* the opening
* mistakes in finishing off and endgames
Here are some cures:
TACTICS
Make sure you know all the basic tactics
http://www.exeterchessclub.org.uk/chessx/pdf/tactics6.pdf
Practise spotting tactics
http://www.exeterchessclub.org.uk/chessx/pdf/tactics6b.pdf
Sign up and rate your tactics at the Lichess Tactics Server
Practise checking whether your tactic is a hit a miss or a trap
Easy:
http://www.exeterchessclub.org.uk/chessx/pdf/mixedpuzzles.pdf
http://www.exeterchessclub.org.uk/chessx/pdf/mixedpuzzlesanswers1.pdf
Harder:
http://www.exeterchessclub.org.uk/chessx/pdf/mixedpuzzles2.pdf
http://www.exeterchessclub.org.uk/chessx/pdf/mixedpuzzlesanswers2.pdf
A course in chess tactics: [PDF] [PGN]
Tactical ideas illustrated with short games in the Italian Game and related openings.
OPENINGS
Know an opening system for White (starting either 1.e4 or 1.d4)
http://www.exeterchessclub.org.uk/chessx/pdf/JuniorRepertoire.pdf
Know TWO defensive systems for Black (starting either 1.e4 or 1.d4)
http://www.exeterchessclub.org.uk/chessx/pdf/JuniorRepertoire.pdf
Know how to play as White against odd black defences (starting either 1.e4 or 1.d4)
http://www.exeterchessclub.org.uk/chessx/pdf/JuniorRepertoire.pdf
[DOWNLOAD all these games in a PGN file for your chess software or play over the examples here]
MIDDLEGAME PLANNING
Know what sorts of squares your pieces like
http://www.exeterchessclub.org.uk/chessx/pdf/strategy6.pdf
Know how to attack and how to defend
Know how to make a plan
content/planning-0
FINISHING OFF and ENDGAMES
Know how to finish off a game by swapping off pieces
http://www.exeterchessclub.org.uk/chessx/pdf/finishing.pdf
Know the basic mates
content/essential-checkmate-patterns
content/vukovics-mates
Practise simple endgames
http://www.exeterchessclub.org.uk/chessx/pdf/iec.pdf
[Updated with links]