Rocks
Not all types of rock are suitable for aquariums.
But it's easy to test: put some vinegar on them, if you see small bubbles appearing they are not suitable.
If that's the case, the rocks contain too much chalk, which can have a bad effect on the aquarium and fishes. It makes the water harder and will affect the pH levels.

Wood
Again, not all types of wood are suitable for keeping submerged for a longer amount of type. It will start to rot and pollute the water or it will keep floating.
Have a look here for some more info: wood suitable for aquariums
In my opinion, it's safer to purchase some wood from a pet store.

Prepartion
Don't cook the rocks and wood! Putting wood in the over could be dangerous!

In stead do this (even with rocks/wood purchased from a pet store):

1. Rinse and scrub the wood and rocks using a brush and tap water (do not use any chemicals). This is to make sure any dirt is removed.
2. Next, you can boil the rocks for a couple minutes. This will kill any bacteria.
   For smaller pieces of wood you can do it too, but if you do it too long it can damage the structure of the wood. You can also pour some boiling water over it a couple times (I did this with my big pieces of wood).
3. The rocks are now safe to add to your fish tank.
   For the wood it's better to keep it submerged in a bucket with water for some times (perhaps a day or 2).
   This will allow the wood to soak up water so it will sink to the bottom once you add it to your aquarium.
   Also, some types of wood can colour the water, by keeping it in a bucket for some time, you won't get (as much) colored water in your fish tank.
   Tip: fill the bucket with warm water (or pour the boiling water over it in the bucket), this will make it go faster.

Rocks
Not all types of rock are suitable for aquariums.
But it's easy to test: put some vinegar on them, if you see small bubbles appearing they are not suitable.
If that's the case, the rocks contain too much chalk, which can have a bad effect on the aquarium and fishes. It makes the water harder and will affect the pH levels.

Wood
Again, not all types of wood are suitable for keeping submerged for a longer amount of type. It will start to rot and pollute the water or it will keep floating.
Have a look here for some more info: wood suitable for aquariums
In my opinion, it's safer to purchase some wood from a pet store.

Prepartion
Don't cook the rocks and wood! Putting wood in the over could be dangerous!

In stead do this (even with rocks/wood purchased from a pet store):

1. Rinse and scrub the wood and rocks using a brush and tap water (do not use any chemicals). This is to make sure any dirt is removed.

2. Next, you can boil the rocks for a couple minutes. This will kill any bacteria.
   For smaller pieces of wood you can do it too, but if you do it too long it can damage the structure of the wood. You can also pour some boiling water over it a couple times (I did this with my big pieces of wood).

3. The rocks are now safe to add to your fish tank.
   For the wood it's better to keep it submerged in a bucket with water for some times (perhaps a day or 2).
   This will allow the wood to soak up water so it will sink to the bottom once you add it to your aquarium.
   Also, some types of wood can colour the water, by keeping it in a bucket for some time, you won't get (as much) colored water in your fish tank.
   Tip: fill the bucket with warm water (or pour the boiling water over it in the bucket), this will make it go faster.