If you have twenty minutes a day for chess, spend most of it on tactics. Below club level, the great majority of games are decided by somebody missing a two or three-move tactic, not by a deep positional misjudgement.
Lichess puzzles
lichess.org/training — unlimited, free, and drawn from real games. It tracks your puzzle rating and feeds you positions at the right difficulty, which is what makes practice actually work. There are themed sets too, so you can drill just pins, or just back-rank mates, or just endgames.
Chess.com puzzles
chess.com/puzzles — a daily puzzle free to everyone, with a limited number of rated puzzles a day on a free account.
Puzzle Storm and Puzzle Rush
Both sites offer a timed sprint — as many puzzles as you can solve in three minutes. It is the most enjoyable way to practise, and it builds the pattern recognition that lets you see a tactic across the board without calculating it.
Try one now
A classic ending every club player should know: king, bishop and knight against a lone king. It is a genuine test, and it comes up more often than you would like. Play it out against Fritz and see whether you can force mate.
How to practise properly
Solve the puzzle in your head before you move a piece. Moving first and looking second trains you to guess, and you cannot guess over the board. If a puzzle beats you, look at the answer, then come back to it another day rather than moving on.