Hi Poppy07! Lets take your questions in order;
For thinning your Oils, try using Sansodor Low Odour Thinners - you can find this at SAA or at your local art shop.
Oils rather than Acrylics? Oils will stay wet much longer (can be several days), but you can buy Alkyd Oil Paint, also known as fast Drying Oils, and still use the above thinners with them.
Acrylics will dry much faster, though you can use Atelier Acrylics, which are "re-wettable", which makes them a little like oils without any of the odour.
Don't use watercolour paper; its too absorbant and will suck all of the oil out of you paints. use either Acrylic or Oil pads, or use Canvas's, which can be bouight quite cheaply these days, often in cut price stores, and these are fine to practice on.
if you really want to have a go at oils, you might be better starting off by buying more useful colours. may I suggest the following as a basic set of 6;
White, French Ultramarine, Cadmium Yellow, Alizarin Crimson, Burnt Sienna, Burnt Umber.
With these 6 colours, you should be able to make almost any colour you want.
If you want any further information, come back to me and I'll do my best to help.
Good luck and very best wishes,
PA & RSC Wigan area