How can I get only the first decimal of a number but without rounding it

You can use this :

sELECT CAST (ROUND(2.488888 , 1, 1) AS decimal(18,1))

sELECT CAST (ROUND(2.4999999 , 1, 1) AS decimal(18,1))

Use Round() function with third parameter.

ROUND(1.88, 2 ,1)

this function has the following parameters.

ROUND(number, decimals, operation)

• number Required. The number to be rounded
• decimals Required. The number of decimal places to round number to
• operation Optional. If 0, it rounds the result to the number of the decimal. If another value than 0, it truncates the result to the number of decimals. The default value is 0

Related link on W3School :- LINK

Just a test snippet of various methods for fun and giggles.

declare @test table (id int identity(1,1), num decimal(20,3));
insert into @test (num) values 
 (1.444)
,(1.888)
,(123456.111)
,(123456.999);

select num
, convert(decimal(18,1), round(num,1,1)) as unrounded -- best method
, cast({fn truncate(num, 1)} as decimal(18,1)) as odbc_truncated
, convert(decimal(18,1), num-num%0.1) as modulusced
, convert(decimal(18,1), floor(num*10)/10.0) as floored
, convert(decimal(18,1), round(num-0.05,1)) as rounderood
, convert(decimal(18,1), stuff(num,charindex('.',num)+2,1,'0')) as stuffed
, convert(decimal(18,1), left(num,charindex('.',num)+1)) as lefted
, convert(decimal(18,1), substring(convert(varchar(20),num),1,charindex('.',num)+1)) as substringed
from @test;
GO

       num | unrounded | odbc_truncated | modulusced |  floored | rounderood |  stuffed |   lefted | substringed
---------: | --------: | -------------: | ---------: | -------: | ---------: | -------: | -------: | ----------:
     1.444 |       1.4 |            1.4 |        1.4 |      1.4 |        1.4 |      1.4 |      1.4 |         1.4
     1.888 |       1.8 |            1.8 |        1.8 |      1.8 |        1.8 |      1.8 |      1.8 |         1.8
123456.111 |  123456.1 |       123456.1 |   123456.1 | 123456.1 |   123456.1 | 123456.1 | 123456.1 |    123456.1
123456.999 |  123456.9 |       123456.9 |   123456.9 | 123456.9 |   123456.9 | 123456.9 | 123456.9 |    123456.9

db<>fiddle here

You can tackle this a few different ways. You could convert to character data or do some integer math.

declare @Nums table
(
    SomeNumber decimal(7,4)
)

insert @Nums
select 1.88 union all select 2.488

select *
    , left(SomeNumber, charindex('.', convert(varchar(10), SomeNumber)) + 1)
    , convert(decimal(7,1), convert(int, SomeNumber * 10) / 10.0)
from @Nums

..fiddle..

select {fn truncate(val, 1)}
from
(
values(1.88), (2.488)
) as v(val)