How to sort array of tuples alphabetically only based on the first item of the tuple in nim?

Use the sort procedure from the standard library. If needed, customize it by also providing a comparison function that defines how the items should be compared to each other, and returns an integer indicating their order.

Simple case:

import std/algorithm

var my_seq = @[("a", "hello"), ("c", "world"), ("b", "to")]

my_seq.sort()

echo(my_seq)

Demo

Example comparing the tuples just by their first element (at position 0):

import std/algorithm

var my_seq = @[("a", "hello"), ("c", "world"), ("b", "to")]

proc myCmp(x, y: (string, string)): int = cmp(x[0], y[0])

my_seq.sort(myCmp)

echo(my_seq)

Demo

Note that I've changed let to var to make the sequence mutable. If you wanted to keep the original sequence immutable, and just based on that define a new one, use sorted instead:

import std/algorithm

let my_seq = @[("a", "hello"), ("c", "world"), ("b", "to")]

let my_seq2 = sorted(my_seq)  # or sorted(my_seq, cmp)

echo(my_seq2)

Demo

All of them output:

@[("a", "hello"), ("b", "to"), ("c", "world")]