DraftStreet/Fanduel/DraftKings are the 3 biggest
I don't think fanduel has golf but DraftKings and DraftStreet do. I don't think any sites have nascar that I know of. Think there are legality issues with this since its a single event or something....
All those sites have MLB/NHL.
I'd try Fanduel first unless you really wanna get into the Golf stuff (which maybe you do with Masters coming up) then I'd go DraftKings.
DraftStreets site is so ugly to look at for me, but they do have a good product.
Also when you start playing treat it like you would treat poker basically. Play low stakes at first. This isn't like sports betting where you bet $2 or $2k the edge is the same, it is much more like poker in that it is not as scaleable so try to play low stakes first while you learn ropes.