For a painter the palette is the most important tool. Without it you are useless. One of my painting mentors said to think of painting in musical terms, and I think this is very useful. Your palette is your instrument, and it has to be able to represent all the notes so you can perform your music well. It also needs to have a system so you know where to find your notes on the instrument. This doesn't mean it has to be able to create all the sounds in the world, but a way of representing all the notes. For instance: a guitar doesn't sound like a piano, but you can play the same notes on them. On your palette the notes are your colors and values. This can be narrowed down to yellow, blue, red and white. "Black" is made by mixing all three primary colors. Which colors you choose will have a huge impact on your spectrum of colors, so I advice working with this continuously and adjusting whenever you feel your palette is not working the way you want it. You'll save a lot of time if you sit down and play with the colors for a while and compare them with other combinations to see if they will fit your needs.
This is a photo of my palette. After years and years of frustration I have come up with this system to keep my colors neat and tidy on the palette so I know exactly where to find the right color at all times. To make it easier to explain I have divided it in to zones separated by grey lines:
1. Primary and secondary colors. Straight from the tubes: blue, yellow, red. Between them I mix green, orange and purple. Mix a maximum of two colors together here. No white.
2. Clear grays and browns. Mix three colors in this area: blue, yellow, red, and blue again. No white here either.
3. In this zone I mix primary and secondary colors with white. I divide this area into four zones. Mix no more than three colors at once here, and only within the area it belongs. You can do with just three zones, but I like to give the blue some extra space because it is more easily made dirty by other colors.
4. This is the ONLY zone I mix more than three colors at once. This means you can mix all your four colors here without messing anything up, and create a spectrum of greys and browns mixed with white.
I keep white in opposite corners of the palette to separate white that is mixed with clear colors from white that is dirty. Whenever the white in the upper left corner gets too messy I move it to the lower left where it will do no harm with the already dirty colors. You can also move left over color from all your other areas here to make space when you need it. In the upper right corner I put dry paint that I scrape off the palette to clear mixing space. I usually scrape my palette clean before or after painting.
If your colors change a lot when they dry, it might be because of too little oil in your paint. Just use a little more oil to make it glossy and clearer. I use raw linseed oil. If it still won't be glossy you can always varnish later.
Hope this was helpful. Happy painting!