demo aisukurimu ha tsumetai desu ^^. wakarimasuka

samui desu yo ^^

samui is a feeling, whereas tsumetai is a property. You would say samui when you feel cold, and tsumetai when something else is cold. For the example you put in your question, it would be samui. If you wanted to say that an object is cold, such as a glass of water, or as Arucard303 said, ice cream, you would use tsumetai.

samui is use when you felt it and tsumetai is for a thing

It depends on the situation. When you talk about weather, "samui" is appropriate. When you feel or touch or eat or drink... cold something, "tsumetai" is appropriate.