It's really hard to stay around $800 if you don't have a monitor or a copy of Windows (those will add at least $200 together), but here's what I have so far:
Side note, check out www.reddit.com/r/buildapc/ which has a ton of guides and good information for first time builders. Watch a ton of YouTube videos, find a friend or acquaintance who has built one before, basically just use all your resources. It's a LOT of fun and is exciting but the first time will definitely take the longest because you're worried about doing something wrong. Let me know if you have any questions!