I've put a few of my tips together for you to live by when you're online shopping.
1. Understand your measurements in both metric and imperial and find the size guide online. I also look for similar built girls on Instagram and ask them what size they ordered as well. If this doesn't help i contact the brand directly to see what they suggest. Also know the conversion for the sizing for international sizes too :)
2. Decide on if you really like it. Will you wear it? do you love it? knowing if you can customize it is important too, like can you take the padding in/out? Not all of this is in the model photos so reading up on your product is a great idea - again i do this while i'm also looking at photos on Instagram of regular girls.
3. Know what styles suit you and stick to them. Just trust me on this, there's plenty of bikinis i've bought and never taken photos of purely because they looked bloody awful.
4. It's so important when you buy any item of clothing that you judge it's comfort. There's no use buying something if you're not going to be comfortable wearing in public or just in general sizing. I usually spend a few hours deciding on a product and looking at it before i buy it. While i'm looking i consider the style of the bikini/swimwear and how it looks on the model compared to how it will look on me. Most websites will actually have their model's measurements somewhere for you to compare to.
5. For swimsuits - to me it's like a complete other ball game, i've never been able to get these right so far because of how short i am vs. my hip size. I'm so worried that they won't fit my hips that i usually order the next size up then its too big for me. Know if you can exchange your purchase before you buy it.
6. Look around for the cheapest products and surf the web for coupons. I do this all the time, not only do i usually get bored a lot but this helps me fill that boredom and i can help others deicde.
7. Ask me for advice if you get stuck, i'm more then happy to help you figure out a size if you get stuck :)