A practical order of operations for your first few puzzles.
New Shikaku players often try to solve from left to right. A better approach is to solve from most constrained to least constrained. That means small clues, edges, corners, and cells that have only one possible owner.
A clue of 1 is already solved. It must be a single cell, so mark it immediately. This may block neighboring clues and reduce their choices.
A 2 clue can only be 1x2 or 2x1. A 3 clue can only be 1x3 or 3x1. If one orientation runs into an edge, another clue, or an already placed rectangle, the other orientation is forced.
A clue near a corner cannot expand in all directions. For example, a 4 in the top-left corner can only extend right and down. That often makes a 2x2 square or straight 1x4 rectangle obvious.
Pick an empty cell and ask which nearby clues could possibly include it. If only one clue has a valid rectangle that reaches that cell, that clue must own it.
Beginner habit: Do not start with the largest number on the board. Large clues have many possible shapes; small clues reduce the board first.
Use a hint after you have scanned all 1, 2, and 3 clues and checked every corner. If the hint reveals a rectangle, study why that rectangle was forced before continuing.