Well it partly depends on the exact situation of course but here's something of a rule of thumb. Particularly as what might be the official standard and what soldiers actually carried on their person might be quite different. There are some things in the field a soldier might decide they can do with less of or without and some they might decide they need more of than would be standard.

For clothing at the start of the war most armies were using cloth or leather caps, woollen jackets and great coats as part of their standard kits in addition to trousers, leg wrappings and boots. Over time as it became clear that head protection was useful against shrapnel, debris, fragmentation ect from artillery shells and the like metal helmets were introduced in most armies. Additionally some armies introduced things like leather jerkins to help keep warm without being overly restricting and off a little protection against bladed weapons.

Gas masks similarly were introduced and improved over the course of the war becoming steadily more advanced to more effectively deal with chemical weapons and the more effective variants of chemical weapons that came into use as the war went onwards.

Usually in the first world war and particularly early war soldiers would carry a lot more rifle rounds than they would do later on when it was becoming common for there to be around one light machine gun per section, at which point soldiers tended to carry slightly less clips for their own rifles in favour of machine gun magazines of various kinds or belts. Certainly 100-150 rounds would be common prior to ww1 as the recommended load in various armies with often a potential maximum capacity some soldiers might be carry could be higher than that while later on soldiers would be more likely to carry closer to 50-75 rounds for their own weapon plus rounds for their section/squad's machine gun or similar weapon such as a B.A.R. This kind of thing is probably more common by the beginning of the first world war as numbers of MGs assigned to a force increased but you were beginning to see that kind of thing as light machine guns became more numerous by the end of the first world war that you'd have at least one such weapon per platoon and possibly two.

At the start of the war grenades were more sporadically used but as trench warfare became the norm initially improvised grenades and then more dedicated designs began to become much more common. By the end of the war grenades were being heavily used and so while regulations usually suggested about one grenade per man particularly units specifically chosen as assault units like the german storm troopers you see a lot of photos of these guys basically carrying as many as they could strap or stuff onto their person. Similarly smoke grenades weren't all that common at the start of the war, and generally smoke shells, mortars and projectors remained a more common way of doing things but some smoke grenades and chemical grenades became to come into use by the end of the war.

Then in terms of other personal equipment you obviously have the rifle itself. A bayonet, specifically a long sword bayonet usually, prior to the war there was something of a concern between lots of the great powers that the combined length of their rifle and bayonets shouldn't be shorter than their enemy's weapons or else their soldiers would have less reach and might be more likely to lose that kind of melee fight. This was particularly notable in cases where countries were also making their rifles shorter to make them easier to carry, handle ect and so the solution was a lot of bayonets for a while became a lot longer than the kind of blade you'd see used as a modern bayonet, which possibly ultimately had some disadvantages for making it all too long for using the confined zig zagging spaces of trenches.

Then there's an entrenching tool, usually the more modern folding designs weren't yet common but many designs allowed the head and handle to be detached from one another. It was fairly common for soldiers to actually carry them into battle in case they needed to try and dig their position in a little deeper and they were also sometimes used for melee weapons as alternatives to things like bayonets. On that subject for trench raids some less conventional weapons were added to the arsenal, side arms which were normally issued primarily just to officers might be used a little more often in addition to weapons like trench clubs and less standardised trench knives handy for close quarters work in the dark.

When attacking an enemy positions while not every soldier would have one some would also be carrying wire cutters, ideally a lot of the time they hoped to fire enough artillery to blow holes in the wire though the imperfect nature of the accuracy and fuses of artillery there may not be enough holes to prevent them from being easy choke points and night raids may require holes to be produced in a quieter manner. For this a large pair of wire cutters was often used.

Generally a soldier would also carry a water bottle and depending on the nature of the engagement and what you were doing you might also have some bits and bobs like mess kits in your back pack, spare socks ect, the kinds of things that mean once you've taken what you're off to go take you can live there more comfortably. I could go into a little more detail on the subject but i'll leave this here for now as i am sleepy.