The solution is to modify the paragraph style you applied to your text. In principle, this is Body Text.
If the main tool bar displays Default Paragraph Style, then you are using Writer in full manual mode which is extremely user-unfriendly, making tuning and other embellishments tedious?
Note that indents and tab stops are independent. Consequently, you can’t synchronise indents with tab stops, even less with implicit evenly-spaced stops.
But since you are mentioning “tab”, I assume you have indented the first line of paragraphs with a Tab to create “alinea space”. You should not do it that way.
Make sure that no direct formatting, DF, (including definition of first line indent) is in effect on the paragraphs you want to change because DF takes precedence over styles. Eventually, Format
>Clear direct formatting
or Ctl+M.
Open the configuration dialog for the style you want to customise and go to Indents & Spacing
tab.
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Before text controls the global paragraph left indent, i.e. the additional left “margin”
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After text similarly controls the right indent
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First line indent is the setting you are interested in: change it to a positive value to create “alinea space” or to a negative value for a hanging indent (“outdent”)
While you are in this tab, pay attention to Spacing parameters: they control vertical spacing above and below the paragraph. The parameters should be used instead of using empty paragraphs to vertically space. Don’t hesitate to set both values. A single Below distance has not the same effect as the same distance as the sum of Above and Below. You won’t see the difference on a text made of a single category of paragraphs (same style everywhere) but it is significant when you have different styles, e.g. a heading followed by text. Setting carefully both allows to achieve results without the use of empty paragraphs.
For more precise and targeted suggestions, mention OS name, exact LO version and save format.