There are several advantages for a beginner to playing the guitar/bass with a strap.
First off, you'll need to get the right strap for you and your instrument. Depending on the weight of your instrument, I recommend a light, medium or heavy leather strap which will usually run between $15 - 60 depending on quality, thickness and width. If you have a really heavy bass, I strongly recommend not only a thicker strap but more importantly, a wider strap. I suggest getting a leather strap because they tend to be more comfortable on your skin.
The length of the strap is another consideration. Many "cool" musicians like Slash or virtually every punk musician wear their instruments very low, which is fine, once you are good! But as a beginner or intermediate player, or if you play a lot of technically challenging music especially with difficult stretches, you'll want to have your instrument higher, around the bottom of your chest.
A strap can be helpful even if you're sitting because if it's properly adjusted you'll be freed up in that you won't have to hold/brace your instrument, the strap has that covered.
To become a proficient player, you'll have to learn to play by feel more so than by sight (over time) and playing with a strap will keep your instrument upright which in turn will force you to play more by feel.
Lastly, I always advise that you practice much of the time standing up. Standing is good for multiple reasons: It really helps you to play by feel, it allows you to move around more freely and standing tends to make you more alert and engaged.