1) اشتريتُ بعض الخضروات لأحضَّر القليل من الحساء = I bought some vegetables to make some soup.
Here I used "لِ" which means "to" and after it comes a verb.
2) اشتريتُ بعض الخضروات لتحضير القليل من الحساء = I bought some vegetables for making some soup.
Here I used "لِ" as well which means "for" and after it comes a noun.
3) اشتريتُ بعض الخضروات من أجل تحضير القليل من الحساء
Here I used "من أجل" which means "for" and after it comes a noun "تحضير".
4) اشتريتُ بعض الخضروات لِكَيْ أُحَضِّرَ القليل من الحساء
Here I used "لِكَيْ" which means "to" or "in order to" and after it comes a verb.
PS : you can use other verbs not only the verb "أحضِّر" Here some of them :
Verb===========>Noun
إعداد <=========== أُعِدّ
I make=========>making
طبخ<===========أطبُخ
I cook=========> cooking
طهو<=========== أطهو
I cook==========> cooking
PSS : As a last point I would like to tell you that you don't need to put "أنا" = "I" before the verb like you did "أنا أشتريت" but it's sufficient to say "اشتريت" which means "I bought" because the "I" is mentioned already. Look here down the explanation :
"اشترى" = "to buy".
"اشتريت" = "ت"+"اشترى" = "I bought".
But it stills correct to say "أنا اشتريت" and as a recommendation from an Arabic native speaker don't mention the "I".
Good luck)