Go to the Delegation Center
Select the stake pools tab to choose a stake pool

Select the pool you want to delegate to, Click on "Delegate to this pool
Tip: Choose a pool that is not over saturated in order to maximize rewards. (AD247 is a great choice! Our pool ticker is AD247)
Few things to note first:
If you want to delegate to multiple staking pools you need to create multiple (Shelley wallets) and split the balance. Then do the delegation for each wallet.
There is no danger of loosing your funds, as your funds will never leave your wallet. You are just telling the network that you are delegating your stake to the selected pool and not sanding that pool any of your ADA.
The cost of delegating to a pool is about ~2 ADA.
There are some parameters to consider:
Cost per epoch: This is a fixed amount of ADA that the pool operator will get and will be substracted from all the rewards the pool creates.
Profit margin: Is a variable reward for the pool, percentage of reward sum that will go to the pool operator.
To play around with this parameters you can check the official rewards calculator
You can also check the website of any pool.
Select the wallet you are going to delegate
Confirm or change your stake pool selection
Use your spending password or connect your Hardware Wallet to confirm the transaction to delegate to the selected stake pool.

Interested in pledging read this article first https://docs.cardano.org/en/latest/explore-cardano/understanding-pledging-and-rewards.html
This for staking, how do I do for pledging?
This for staking, how do I do for pledging?