US Treasury Bonds (known as T-Bills) can be bought from the US government directly. Here are some links to help you get started:
https://www.treasurydirect.gov/
https://www.treasury.gov/services/Pages/Savings-Bonds.aspx
Other securities, such as stocks corporate bonds, typically aren't available for purchase by the general public from firms directly, so you need to go through a marketplace (an exchange like the NYSE or NASDAQ), or you can have a broker such as Charles Schwab get them for you (of course, you will pay them fees however).
If you want something a bit quicker, get a Fidelity, E-Trade, or Ameritrade account and they'll give you access to the market so you can buy and sell to your heart's content.
Hope this helps, and best of luck! If it's your first time investing, it's a general rule to follow that you should not invest more than 10% of your total savings. You must also be prepared to hang on for the long-term, and recognize that returns of 2-3% are considered normal, even if they seem small.
