How do u train to go beyond fishing 300?

You need to goto the Zangarmarsh.  Juno Dufrain
Fishing Supplies has Master Fishing - The Art of Angling
Requires Level 45
Requires Fishing (275)
Use: Teaches you how to be a Master Fisherman, allowing a maximum of 375 fishing skill.

He is located 78, 66 in Cenarion Refuge.

Ty for your concise and explicit answer.  most helpful.  :)

for the above answer..it was spot on and thanxs for this it helped lots

Absolutely