Take 3 steps back, go to yoru chamber of commerce and make an into into running a business. If yuo can not even envirion how people pay you (although you likely have paid a lot of companies in your life) - you are wrong running a business. BETER LEARN. DOnt ask in forums here, I am sure your chamber of commerce has nice intro courses.
In general, yuo receive payment by telling customers to pay in one of some specific ways. I am sure you have heard of such advanced financial concepts like "bank account". You should get one. People then can "transfer" money to it, sometimes called "making a wire transfer".
If you are smart enough to type your bank coordiantes (account, bank name) on an invoice without making an error, the customers payment will even arrive in your account.
Some businesses accept cash, but this is mostly shops (i.e. not really applicable). Checks are not really used anymore in most civilized countries outside the US which still seems to have a hack for mailing checks. I have not dealt with a check in 30 years or so. I also would not deal with any company demanding a check, but then I am not in the US.
Paypal is an option, too - b ut be carefull to google around 15 minutes to see about the pitfalls it has.