Meh. You use "and" and "then" too much. Too short. Also, put a space after every period/exclamation point/quotation mark/comma/question mark. Otherwise...
He was in one of the smallest towns in Hoenn, Littleroot.
Not relevant. The rest of the paragraph is about Jake's dreams and the TV show that's making him think about them. Don' stuff in random information where it doesn't fit.
Suddenly he heard his mum calling so he turned off the tv and headed downstairs to have breakfast.
'Bye Mum!' Jake shouted back happily.
What happened to breakfast? How is Jake one second running downstairs to eat and the next leaving the house?
'I'll take this one Professor.' Jake said picking up the pokeball that was the third from the left.'Aaah, you choose Pichu as your first partner Jake?' Prof.Birch asked eyeing Jake carefully.'Yup!' Jake said almost hugging the pokeball.
How does he know it's Pichu? And even if he did, why does he choose it without even looking at the others? Shouldn't he be sending them out and looking at them rather than just grabbing one and deciding to take one?
'I have a Pokemon!' Jake said releasing the Pichu from its ball.'Ooh Its soo cute!' His mum said as she picked the Pichu up.
What does Pichu look like? You should always describe the Pokemon, especially in a place like this. A nice paragraph telling just how adorable Pichu is would work perfectly here, right after Jake sent it out.