There are no general restrictions on Shield Guardians using magic items
In Chapter 7 of the DMG, in the section called "Magic Items" the book lays out the categories and general rules for using each category of magic item (I'm not going to list them all here).
There are some limitations listed (rings must be able to be worn on an applicable digit, and you must be able to read a scroll, for examples), but none of them restrict the use of magic items by Shield Guardians or constructs in general.
Specific restrictions will apply (eg spellcasting attunement)
However, many magic items have restrictions beyond their general rule so you must check with each item you want the guardian to use. For example, since shield guardians are not spellcasters they will not be able to attune to (or use) items that require the user to be able to cast spells. Many cases may require your DM to make a call about whether the particular item can be used.
Note that any items attuned to it count as attuned to your PC (in AL)
Any item attuned to an NPC under your control (such as, but not limited to, familiars, beast companions, simulacrums, conjured creatures, hirelings, lickspittles, etc.) counts against both your character’s limit of three attuned items and the character’s permanent magic item count.
This rule doesn’t imply that such creatures have the ability to attune
to magic items. Whether or not a mindless undead creature—for
example—can attune to something is subject to DM discretion. (DDAL FAQ v 7.1 p. 8)
So, make sure you are abiding by your 3 item attunement limit.