First, I assume you're in UK certain of your identification. If not, do not read on!
Otherwise, Andy's right. If you are committed to spending time and effort on it, you need a tank, outside, not in the sun. Lots of earth, stones and plants, and make sure there are dry parts and holes to hide in. A pool is good. Tank at least as long as snake. Good lid, such as a sheet of glass or mesh, well weighed down. In winter put it in a garage or somewhere else cool.
Feed it on live frogs or goldfish every couple of weeks. (Everybody's got to eat someone). Dead food will not do. Prey up to twice the width of the snake's head. Not toads.
Handle it. Stinky stuff on your hands the first few times, but then it'll get quite tame.
Best to do what Sawyers says -- keep it for a few weeks, then let it go, so it can find friends and breed. Not in open field though -- they need shelter. Ideally a tall grassy place near water.
If it's very large (say over 60 cm), it'll be a female, and may be needing to lay eggs quite soon. You can't give her the right conditions, so I suggest letting her go now, near a good big compost or manure heap which will not be disturbed this year.
Don't worry, RUKiDdiNG, it's not deadly, except to frogs etc. (The names of animals don't always tell you where they live -- though in this case it does...)
In UK it is legal to keep grass snakes, but illegal to injure or kill them.
I've kept quite a few grass snakes, and they do make reasonable pets -- but they are of course always going to be happier in the wild.
Finally, "miss", if you don't feel completely confident about any of this, phone your county wildlife trust (in the book).
Good luck!