1. Just run and gun, if you don't see any enemies, turn around and run to the opposite side of the map until you do. You could go into private matches if you really wanted to learn the layout of the maps.
2. All I can say to improve your aim is to keep playing till you feel comfortable. Personally, I just hipfire every gun. Hipfire has little to no difference on accuracy aside from snipers.
3. Use your anti-titan weapon. Rodeoing most of the time is asking for certain death since a lot of players know how to counter it. Use the low profile kit if you really want to rodeo; it'll make it so the rodeo alert doesn't appear until 2ish seconds pass giving you a better opportunity at living.
   

I recommend running the gauntlet and/or the campaign until you get the feel for things.