Pots and containers - advice really needed please

[Aperolspritzer]

I have never planted pots or containers before so am clueless. I bought 2 'mini' firs which I need to put into nice containers for a patio and 2 of those dark red grass spidery plants for nice pots on decking.

So - do I just out the trees into large pots with just soil or do I need gravel/drainage holes/anything else??
As for the plants - I know they need drainage - but if I make holes in the pots won't the water leaking damage the decking? Do I use pots inside larger container?
Apologies in advance for extreme lack of knowledge - I'm hitting the garden centre tomorrow so any advice tonight will be very welcome.

You need to plant your plants in pots with drainage holes, because if you don't the soil could become waterlogged and the plants will drown because roots need air. Any pots you see without drainage holes are intended to be decorative, to cover smaller plant pots.

If you are worried about damage to the decking, either buy one of those decorative pots to catch the water that runs out of the plant pot, or use a saucer. In dry weather, it's good to have something that retains the surplus water for a while, but you'll need to check that the plants aren't sitting constantly in a pool of water.

PS. Yes, a layer of gravel , crocks (ie broken flower pots) or broken up polystyrene packing at the bottom of the planting pot is good, because it stops the drainage holes getting clogged with compost - although recently some gardening magazines have been saying you don't need to do this.

disk cut from cardboard at bottom of pot/container lasts easily for a year or two and stops soil from falling out.

Yes to polystyrene, especially for larger pots you intend to move around. Often you don't need that much soil and the weight of the pot is lower when partially filled with polysterene. It does help with drainage which is essential, see Maud's post above.

Yes, polystyrene is my favourite method because for shallow-rooted things in big pots - especially those potted in John Innes - it saves money and weight.