I have a lot of things to tell you

I have a lot of things to tell you.

In my opinion.... I have a lot of things to tell you. ( Here you are using "" a"" so, suitable"" lot) Next: I have lots of things to tell you( a "" not use here ( focus on singular and plural...

Both are fine and very common! I have lots of things to tell you. I have a lot of things to tell you. You'll hear both all the time from native speakers :)

In realtà, se togli la proposizione, puoi usarlo anche con la forma plurale che hai scritto.