How Far is too Far before it's a sin?
First of all, that may be the wrong question to ask. When you ask this, you are distorting God and making him out to be this guy who is going to throw a lightening bolt at you when you cross a certain line. It's not the best for your spiritual health. Instead of asking "How far is too far" you should ask "How close can I get to Jesus". We want God to give us all the concrete answers so we won't need him, but we are promised the Spirit, and especially in dating relationships, we need to rely on him to lead us, guide us and to teach us.
When you're dating someone, you aren't under a covenant, therefore they aren't yours!
When you think about the person you are dating as someone else's future husband or wife, that changes how you treat them. You will honour them physically in a way that on their wedding day, if they don't marry you, you would be able to shake their future spouses hand with respect.
This Amazing Gift
Biblically speaking, all those physical things were created for good and joy in a marriage covenant. You are supposed to enjoy your spouse, and sex is an amazing gift of God to be enjoyed, but what happened was we distort it and cross the line before we marry. God created sex as a glue to make the covenant stronger, to bring together and to make one flesh. But when you cross that in a dating relationship, it creates a bond and connection that clouds your judgment to really get to know that person in the dating context.
Set Yourself up for Success
Avoid tempting situations. It's a struggle because you love that person and are attracted to them, but lean into the Spirit and honour him. Don't put yourself in a compromising situation. Try to avoid being alone in the car late at night, or on the couch together. Aim to hang out in groups or to be in public situations to avoid the temptation of crossing a line that may cloud your judgment.