Once your inventory is full of shrimps 
, you can start a fire
# Left-click on the tinderbox in your inventory so that it is
outlined in white. Then left-click on the logs to use them with the
tinderbox. You will then place the logs on the ground and attempt
to light a fire.
# Alternatively, you can drop the logs on the ground and left-click
on the tinderbox so that it is outlined in white. Then left-click
on the logs to use the tinderbox with the logs.
(From www.runescape.com)
To cook using a fire, left-click on the item you would like to cook
in your inventory so that it becomes outlined in white, then
left-click on a fire.
The 'Cook X' option allows a player to cook multiple foods (of the
same type) without having to select the item repeatedly. When you
use a food item with a Cooking range or an open fire, a screen
similar to the one below will appear.
After this screen appears, right-click on the object shown and the
option to cook one, five, X or All items will be shown. To cook one
item, select the 'Cook 1' option. To cook five items, select the
'Cook 5' option. To cook X amount of food, select the 'Cook X'
option and type in the amount of food you would like to cook. To
cook all of the same type of food in your inventory, simply select
the 'Cook All' option.
[Extracted from Runescape.com: Cooking is useful, because eating food
restores lost Hitpoints. As someone new to the skill, you may find
that you may often burn food that you try to cook. The more your
level increases, the less food you will burn. As your Cooking skill
increases, you will also have access to a wider range of foods that
you are able to cook.
You can cook food on an open fire or on a range. ]
You can throw away food that are burnt and net for shrimps and cook
them again until your inventory is full of cooked shrimps
Go to the General store
Northwest of Lumbridge Castle and sell those
shrimps.
Get up some logs if you feel that its not enough later on and go
back fishing and sell em until you feel that you have enough OR you
can fish better fishes
(Information and pics all
taken from www.runescape.com)
