Hi @megean23
First step is making sure your computer meets the requirements for the game to run.
The buying/downloading part is easy, specially if you buy it through Origin. You need to download and install Origin program, then log into your account and the bought game will appear in your "My games" section. You can see your expansion packs if you right click on The Sims 4 cover and choose "Details". To install the game, just click on it.
About what to start with... open the game, create a new game and the game will lead you. It will open the Create a sim first, so create your sim. Once created, you will see the map to choose a house for your sim or sims. And then... do whatever you want! No rules 😉
