You can say '我好想你' - wǒ hǎo xiǎng nǐ
我 (wǒ) means I
好 (hǎo) means very or very much
想 (xiǎng) means miss
你 (nǐ) means you.

If you want to say 'I miss you', you can say ''我想你' - wǒ xiǎng nǐ

Or you can just say '想你' - xiǎng nǐ , meaning miss you.
If you want to say "I really miss you very much', you can say '我真的好想你' -- wǒ zhēn de hǎo xiǎng nǐ
真的 (zhēn de) means really