How Can Hand or Garden Tools be De-Rusted?

Remove excess rust with steel (wire) wool soaked in lemon juice or vinegar which are mild acids. Rinse the tools and allow them to dry. You could soak the tools for a period to remove excess rust.
Another alternative which definitely works is phosphoric acid. This is often a constituent of rust removing chemicals and can also be bought in gallon containers from companies which supply cleaning chemicals. It is a powerful acid and can also be used for removing limescale from surfaces. Usually it is diluted with water before use. The objects being cleaned are submerged in the solution and should only be left in the acid for the bare minimum of time to dissolve the rust, otherwise the acid will start to dissolve the underlying metal! I have cleaned an old vice pillar drill vice belonging to my grandfather which developed a thick layer of rust after being exposed to dampness in a shed for over thirty years and the acid completely removed the rust.
Once you remove the rust, rub some oil into them and wipe off any excess. This will help to prevent future corrosion.
Sand can also be used as an abrasive for removing rust from tools.
If you wipe tools with light oil after use, it helps to prevent condensation and subsequent corrosion. Guns and tools were traditionally given (and probably still are) a coating of oil after manufacture and before packing to protect them from dampness and rust.

Can an Oil Stain be Washed Out of a Cotton Shirt?

Any of the well known hand cleansers used by auto mechanics or similar are excellent for cleaning oil stains from clothes or even worse carpets. The stuff I use on my hands after I do work on my lawn mower is excellent and seems to remove even the worst grime. Some of these cleansers contain hydrocarbon solvents which may remove colour from clothing, so try using some on the tail of the shirt first to see what the affect will be. Wash the shirt in your washer after removing the majority of the stain. Cotton is a somewhat porous fabric compared to to synthetic fibres which are more smooth. So the oil content of the stain will be removed, however you may not be able to remove all the stain, especially if black, waste oil is involved.  However it may become less obvious after further washes.