Hi!
Welcome aboard!! Glad you are enjoying your guitar journey and the website here. Knowing piano and having some music fundamentals under your belt will give you a good head start.
1) When you are starting out with guitar, there are mechanical things you need to work on which will require building up some physical strength in your hands, fingers, and some calluses on your finger tips. It's important to practice regularly in the beginning, but not so much that you are hurting yourself. Think of the marathon runner - when he starts training, he's not going out running 10 miles a day right off the bat.
2) Definitely devote time to warm ups. 5, maybe 10 minutes should be good to warm up. Warming up doesn't mean jumping right into practicing your scales at max speed. Stretch out your fingers and your wrists. Play some scales slowly. Strum some chords.
3) In the very beginning, there are a lot of fundamentals to work out. Unlike piano, there is more of a ramp up with guitar. Frist off, you have to learn how to tune it. You have to learn how to fret notes, how to pluck or strike the strings, etc. As soon as you can, I think it is important to start learning songs because that is what will motivate you and hold your interest. Don't beat yourself up though if it takes weeks to get to that point.
The important thing is to try to be patient with your self and take your time. If you stick with it with some daily discipline, you'll do very well and will be playing songs and strumming chords before you know it.
Hope that helps.
