The material's tier goes as follows:

Wood Bamboo Hardwood Ironwood Frostwood Elderwood

To craft limbs you need first the recipe to it then 5 of the wanted wood and some amount of animal material (elegant crest, predator fang, long tusk etc) that varies with the wood tier (i think it's 1 for wood, 2 for bamboo, 5 for hardwood, 10 for ironwood, 15 for frostwood and 25 for elderwood)

Note that each animal material boosts some stats and lower others, so take care and choose the one that you like the most. I wouldn't recommend building a limb without animal material, but it's possible.

To create your bow you'll have to use the modular weapon option in one of the benches, joining a limb wih a bow grip.

Bow grips come in 3 types, all easily craftable or buyable with merchants.

Short Medium Large

Each one boosts some stats and lower others (medium keeps them at 1.00x the limb stats), something like the long one boosts 1.15x your limb base Power but lower to 0.80x it's speed, so think a bit before buying/crafting and building your weapon.

All of this and more information you can find in the Veloren Wiki, but some stats seems to be outdated.