There are a few different ways to express missing somebody.
[会]{あ}いたい
Generally saying that you miss someone (hence, you want to see them). This is the most casual option available to you.
会いたいな
I miss you.
**I think this is your safest bet.
しばらく会っていないから、会いたいな〜
It's been awhile, I miss you!
[恋]{こい}しい
This is a really strong missing; a yearning for something that you can almost feel. Maybe it's painful, maybe it's nostalgic.
おじいちゃんが死んでからもう一年が経っているなんて信じられなくて、とても恋しいです。
I can't believe it's been a year since my grandpa died; I miss him so much.
四年間外国に住んでいて、お母さんの料理が恋しくなってきました。
After living abroad for four years, I've really begun to miss my mom's cooking.
[寂]{さび}しい
This emphasizes the result of your missing them (you feel lonely). Not a native, but it feels relatively strong to me.
お願いだから、行かないで。君がいなくなったら寂しくなる。
Please don't go - I'll miss you.
明日帰国すんのか?寂しくなるよ。
You're going home (back to your country) tomorrow? Man, I'll miss you!
After you (plan to) meet?
And then, once you arrange a time to meet or meet up again, you probably want to express that too, right?
(君に)会うのが待ちきれない!
I can't wait to see you!
会えて嬉しい!
I'm so glad to see you!
来週の木曜日?待ちきれないよ!
Next Thursday? Can't wait!