Know your measurements
Underwear is the base for your whole outfit, therefore you want to pick something that has a comfortable fit. Okay, so it's not just even about the comfort, the right underwear can accentuate your figure in all the right areas to make sure your outfit stands out for all the right reasons. Sizes vary from store to store so it's very important to double check the sizing guide. Especially when ordering online, you can't you know... physically hold the product in your hand to see exactly what the size looks like. This is how I ended up ordering door knobs instead of dishes, people!
Know what styles you like
Underwear comes in so many different styles that it's important to work out which ones work best for you. Do you like high rise panties, thong bodysuit shapewear or sports bras? What works best with your lifestyle? If you're always at the gym, no matter how much you like lacy mesh bras, you're probably going to want to invest in a sports bra because they offer that extra support. You gots-ta also think about what you're hopping to achieve.... shapewear is great if you want to show-off your figure or like to wear body-con dresses.
Think about the fabric
I don't know if this is true for everyone, but a lot of the people I know seem to have a preference of fabric... I don't particularly... but, there you go. I think fabric is important though when thinking about durability, because certain fabrics can lose their shape after two or three washes, which isn't ideal for underwear. It's good to do your research before investing in anything too expensive, unless... you know, you have a money tree. Also, make sure you read the washing instructions, most people just ignore those and ruin their clothing in the process.
How will it work with your clothes
Leave the best to last they say and this is probs the best point. Circulating back to point one... the whole point of underwear is that it's the base for your clothes to sit on... unless you're trying to impress someone... then it has a whole different function. Shape wear can be great for smoothing out your shape, like the best shapewear for lower belly pooch. Anyway. Yep, think about the clothing you're going to wear it with. If you're wearing light coloured clothing, dark coloured unwear will show through. This can be a cool effect if intentional or a reason for judgemental people to gossip about you. You're going to want something with detachable straps for strapless dresses. For tight fitting clothing... it's best to pick something without a texture.
And that's all folks.