Birthday

Some days ago, it was my father's birthday was.   or  My father's birthday was some days ago 
My sister and I wrote letters and made a surprise. so we could make him [my father] happy, . .
[We could make my father happy, and we were also happy.
[We also were happy.]
After this, my family ate a cake.
The cake was peaches' cake, and it was delicious.
[It's very delicious.]
I thought this party was a successed and made my family very happy too.

or   I thought this party was successful and my family was very happy.

A good effort.  Try linking short sentences, using commas ,  for a more natural style.