The weekend

The last weekend I went to the beach with my friends. We were really happy and, when we arrived there, we put our towels on the sand. The sun shined* and it was very hot. I went to the sea, the water was cold and the waves were tall. I was in a good mood but I saw a jellyfish near me. Oh no! Therefore, I went back to the sand again and I didn't go to the water in all the day. I was frightened!

The weekend

The Last weekend I went to the beach with my friends. We was were really happy and, when we arrived there, we put our towels in on the sand. The sun shined shone and it was very hot. I went into the sea, the water was cold and the waves was were tall (or high). I was in a good mood but I saw a jellyfish near me. Oh no! Therefore, I went back to the sand again and I didn't go into the water in again all the day. I was frightened!

Very good Alicia. My corrections are in red and my suggestions to sound more natural are in blue/grey.

"we put our towels in on the sand" - You would say on the sand if you are simply placing them on the ground. If you were covering the towels with sand, perhaps to stop them from blowing away, you would say in the sand. For example:

We put our beach chair on the sand. (It sits on top of the sand).

We put our beach umbrella in the sand. (Part of the pole goes into the sand).

The difference between shined and shone is very subtle, and difficult to explain. Both words are the past tense of shine. In general, it's probably safer to choose shined, unless there is no object or adjective following it. For example:

The sun shined all day.

The sun shined brightly.

The sun shone.

Of course you could also say "the sun was shining" as James suggested.