I was in the same position before.
I used to play Jungle/Top, but I found that I stopped enjoying top-lane as the meta changed. As such, I decided to change my secondary to mid-lane and had to find a few reliable mid-laners for when I needed to fill the role.
Personally, the following champions are the ones who I believe would be good picks to learn and who I would recommend you try.
A very safe mid-laner due to her shield, CC and range. Even if you fall behind, you have the utility to remain extremely useful and if you get ahead, you can burst down most targets rather easily.
Very mobile champion with pretty much everything you would want in a mid-laner (mobility, burst, CC, waveclear, sustain). She's pretty much always a good option if you know how to play her and she's never really been weak since her rework.
Ziggs is great at zone-control and poke/waveclearing. These traits make him a monstrous champion when fighting in the jungle or during sieges while his range and safety make him very difficult to deny farm once he has some mana regen.
High learning curve but she is very flexible, provides great utility and sustained damage, and has a fantastic teamfighting ultimate. She's great at trading in lane and farming, letting you safely and reliably establish a lead even if you aren't killing your opponent.
A somewhat less common pick, but very powerful when played properly. He has the tools to handle pretty much everything in his kit and he can adapt to most match-ups. While you don't always want an AD mid-laner, I'd recommend having him as a back-up champion for when you need one (i.e. AP top and AP jungle).
Very flexible, powerful and has a decent amount of utility. His waveclear and poke are fantastic, he has good sustained damage, and his in-fight durability and mobility are pretty nice.
High utility, high burst and sustained damage, great waveclear and her wall is extremely versatile. Her main issues are her mana costs and her lack of mobility, but if you can work around that she's a very strong pick.
Personally, I lean towards Lux and Jayce myself as my main mid-lane picks.
While Jayce is a recent addition, I'm having fun learning him and find him to be rather powerful.