I'm new to D&D and I need help with my character sheet. This is the character sheet I'm looking at:
One of the things I need to fill out is "vision". How would I go about doing that?
The standard official character sheet doesn't have a "vision" section, but the "Alternative 2" one from that page does (next to a little icon representing an eye). I understand your confusion, because vision isn't called out as a special game term anywhere in the rules (and that icon, like all icons on this character sheet, is idiosyncratic to this sheet, not used throughout the rules).
However, it's a reasonable section to have, because it's a long-standing idea throughout many editions of D&D that different character races have different types of vision. Particularly, many races have Darkvision. From the basic rules:
Many creatures in fantasy gaming worlds, especially those that dwell underground, have darkvision. Within a specified range, a creature with darkvision can see in darkness as if the darkness were dim light, so areas of darkness are only lightly obscured as far as that creature is concerned. However, the creature can't discern color in darkness, only shades of gray.
So, presumably in this section you would put whether your character has darkvision, and to what distance that darkvision works. This is listed in the "Traits" section of each race — for example, for dwarves:
Darkvision. Accustomed to life underground, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.
so you might write "darkvision (60 feet)". If your character is a Drow (an underground-dwelling elf subrace), you would write "darkvision (120 feet). Or, you might have darkvision from a spell or other source, or something might alter the details. For example, there is a magic item which doubles the range of your darkvision if you have it already.
As noted in the basic rules linked above, other possibilities might include "truesight" or "blindsight". These would be much more rare (but not impossible) for player characters.
Presumably, if you don't have any special vision abilities, you would put "normal" or just leave this section blank. You could ask your DM, although really the point of the sheet is to provide you with the information quickly, so you should do something that makes sense to you (as long as it's clear to the DM, too).