Choose a wallet program, several are well suited for receiving BTC safely. If you are directly receiving BTC for transactions be sure to generate a new wallet address in your wallet program for each transaction. This is for your protection. It is recommended to then transfer the BTC from this working "soft wallet" to a "cold storage wallet" if you intend on saving the bitcoin or to an exchange.
Here are a few different bitcoin wallets to try:
Bitcoin Electrum Wallet
One nice thing about BTC is that you don't need to have a computer on hand when you recieve it. Many wallet programs have QR codes they will generate for your "public address" or "BTC Recieving Address." If you print out several of these, or the wallet adress itself, at a time you can keep them handy for when someone wants to pay with BTC. Always remember to write down your wallet seed! If you lose your seed and are unable to access your wallet no customer support exists to retrieve the funds, it is the same as losing cash at that point. However, the benefit to this is the assurance that your funds are controlled by you and only you, for you are the only one that should ever know your personal seed.
With your wallet public addresses on hand at the time of you need to conduct a transaction, have your client send the funds to the wallet address you assign them for that transaction. Make sure they have the transaction urgency or fee set to the lowest setting. This will mean less fees in trade for longer waiting times. Please expect to wait up to 24 hours to recieve funds. If you trust the person feel free to proceed with your service or product immiadetely, if not this is best suited for large purchases with the buyer being informed that the purchase will be conducted once the transaction is "finished confirming, which may be up to 24 hrs"