AS in any programm going to Files > NEW will open a new file and a window with all the setting that your work will have will appear as the size, name, color mode, background standar initial color etc
click in OK and u will have a new window where u can start to work. cuz its more easier for work i will use an easy lineart in this Tutorial.
BEFORE ANYTHING its recomendable to create an estructure in the layer windows.. what this means? in this work you wont use more than 8-10 layers but as more you advance and progress you will use more and more layers so u need to order them with the folders...
click on the folder icon marked in blue and a generic folder with the name of "group 1" will appear with 2clicks on the name u will be able the change the name of the folder/group. i like to make a group for the background and a group per character AND on the character´s group create sub groups for the skin, clothes, items, hair, eyes and in more advanced works i create in the subgroup anothers specific groups for shadows and lights.
Also for starters is good to make a copy of the lineart and put them bellow the original and set this last one as "multiply" on the layer blending menu WHY? when you start to color sometimes you will color over the lines of the scan/linearts and this will cause some errors laters, so setting the copy as multiply will guarantee you that the lines will stay black.
NOW that u done all this u can start to color, to begin with as this is your first time lest make it easy and enjoyable =)
select the magic wand and chose an area of the lineart, it can be for example the SKIN, the HAIR etcetce, once you select the area that u want to start to work first open the subgroup and create a new layer ( the icon to create a new layer is on the right of the groups icon on the layers window)