Most Field Guides have gilled fungi grouped by Spore Colour. So the first step is to find out the spore colour.
The best way is to get a spore print by placing a cap on a sheet of paper and leaving it till the spores drop (maybe overnight). You can cut of the stem and place the cap flat if you like. For small fungi you can make a hole in the paper and put the stem through this into a pot of water (it prevents them drying out).
In 'the field' the gill colour can be a guide to spore colour (providing you have mature specimens). But beware, in most species the gills start pale and change colour as the spores ripen (eg: the cultivated mushroom's gills change from pink to greyish and finally blackish). Again, ripe spores that have fallen from the gills sometimes accumulate on the upper part of the stem so that you can sometimes tell their colour.
The basic colour groups are White, Pink, Brown and Black. But the is a quite a range of colours you will come across: pure white grades into cream and yellow then pink, ochre to yellow-brown, rusty-brown, snuff-brown, purple-brown and black (a few even green).
For example, Amanitas have white spores, Russula species will have white to dark yellow spores depending on species, Entoloma & Pluteus have salmon-pink spores, Conocybe have yellow-brown spores, Cortinarius & Galerina rusty coloured spores, Inocybe snuff-brown spores and Coprinus (black).
You will find that spore colour is a big part of identifying your collections to Genus.
I do not know for certain even which Genus your species is in. But as it is late Spring and it is growing on litter I suspect it may be an Agrocybe species. The gill colour is about right (mature pores of this Genus are greyish-brown which seems to fit your image of the gill colour). I'm not able to see details of the stem, but most of the large Agrocybe species will have a ring or 'ring zone' on the stem or sometimes the veil(ring) will remain attached to the cap edge. So I'm thinking of species such as Agrocybe praecox (although I'm not convinced!).
