Hi there iloveeveryone, and thanks for your question. Firstly, it is okay if you are excited and a bit nervous about trying this new thing, if you're still able to insert your finger in your vaginal canal, you'll still be able to get a sense of your cervix height. Your cervix's position can change a bit during menstruation, so it is best to measure between days 1-3 of your next period, so you can find it's position when you'll be using the cup.
Being relaxed will make it easier to insert your finger so that you aren't tensing up your muscles, however this doesn't mean you need to be the calmest you've ever been. Just like you don't have to be completely calm to insert a tampon or similar. It's also okay to try, and if it's not working, try again later. Sometimes having a first go at it will make part of the experience a bit less "new" and help dispel some nerves.
What can help (also for inserting your cup the first time when you get it), is to put some lube on your finger to help it slide in. Measuring during your period will also mean that the inside of your vaginal canal will be a bit more lubricated internally too (unless you have just removed a tampon and feeling a bit dry).
There are some great resources about menstrual cups on this YouTube channel, including comparisons and tips on insertion, removal etc. here:
Also, it is totally okay if you try a cup and it's usable, but maybe not the perfect one for you - it'll still work and then you can look at what you'd like to try differently with the next one. If you have any other questions about cups, we're very happy to answer here