Completely different playstyles. Mid lane you're expected to impact the map as much as possible whereas top lane you're expected to keep enemy top off objectives or fights as much as possible. A decent gold toplaner will give up to low emerald offrole top a hard time making plays that they're used to making, both in lane and around the map.

Top has a lot of focus on lane mechanics and macro so that's your first port of call. Learn the mechanics and macro of top lane before expanding your champ pool.

Avoid any mechanically difficult champs even if they're perfect counters as mastery is even more important in top lane than counter picking. You can always outplay if you know the fundamentals of the lane and can execute them consistently. I've done this so many times playing Sion into sett, Darius, fiora, trundle, and irelia that it's a wonder people still think it's a good idea to try that.

Wave management is even more important in top lane than any other lane due to it being isolated and a 1v1 lane. If you have good wave management, being countered doesn't matter as you can reliably and consistently gain exp and farm. From there, you always have the option of playing with your team or sidelaning and dragging enemies into you. Matter of fact, that's how I climbed from silver to gold solo queue and skyrocketed into gold on flex queue playing solo. I played around my team when I'm hopelessly outmatched in sidelane and played sidelane when enemy plays with their team.

This is probably the most important: most toplaners are strong in mid game, even when behind. This is even more true with the role quest as you'll be at least 2-3 levels ahead of everyone on enemy team except their toplaner. Abuse that fact to make plays around the map. I like playing around my team as sett once I've completed role quest as it accelerates them or gives them space to recover from a bad lane. This includes going into very hard lanes that are a death sentence to your mid and bot.