If you are a complete novice....Some simple things to check are.
Metal in the oil, or metalic residue is always expected, it depends on what degree/percentage.
Lift the gear set out, check all roller bearings for noise.
Look at each gears dog teeth (the little teeth on top of the gear and under the Gold colored Syncro Rings).
The Dog gears should be shaped like this ^ (allow for wear of course) some rounding is to be expected.
You should also inspect the sleeve (the round thing that slides over the gold things that slides over the dog gear) , it will also have teeth that should look like this ^.
Wear of the Sleeve,Syncro & Dog Teeth will typically be the main reason a gearbox is hard to select gears.
Bear in mind this is a very basic visual check only.
Good Luck !