Disclaimer: This is not an investment advise. The information below provided for instructional purposes only.
Get some BNB SmartChain tokens
To buy anything on pancakeswap, you need to have BNB SmartChain tokens (aka SmartChain). The easiest way to get them is to first buy BNB tokens on BinanceUS, then transfer it to you TrustWallet. If you’re not in US, you can probably buy BNB tokens directly from Trust Wallet but the fees are high.
Also note that Binance has a lock period for withdrawal, so you may need to wait few days before you can transfer BNB to your Trust Wallet.
Once you get BNB in your Trust Wallet, swap them for Smart Chain Token by clicking on your BNB balance, then on “More” (that is “…” on the right next to Copy), and then “Swap To Smart Chain”
Maybe, there are ways to get SmartChain tokens directly w/o getting BNB first and then swapping, but I’m not sure.
Change Blockchain to Smart Chain
Your Trust Wallet could be on Ethereum blockchain (or some other chain). You need to change it to Smart Chain before using PancakeSwap.
Simply click on a chain icon in a top left corner and switch to Smart Chain.
Connect TrustWallet to PancakeSwap
In your Trust Wallet, go to “Browser” tap and paste this address
https://pancakeswap.finance/#/swap
If you do not have “Browser” button in Trust Wallet, you need to go to your main browser and type “trust://browser_enable”.
This will enable the button.
You will be asked to connect to your wallet: 
After that you can select what currency you want to buy how much you want to spend, and then click “Swap”. 
You’re done!
If the currency you’re looking for is not available in the drop down, you can search the currency by its contract address. For example,
NFTART has 0xf7844cb890f4c339c497aeab599abdc3c874b67a address.
SafeBankToken has 0x52e22D2480bC39F4aB13d4788014AEc3195bbCf7 address.
You just need to put this into a search box, and it will find the currency for you.
If you want to buy NFTART token, see here: http://allstuffaround.com/index.php/how-to-buy-nftart-token/ but this is mostly a duplication of this information with few more screenshots.