There are loads of threads about this so take a look around to get more tips there too.
My tips are -
1. Make sure you are fully hydrated and full - you do not want to get hungry or thirsty.
2. It is a LONG - arrive with enough time to use ALL the allotted time (if you need it) - you do not want to be worrying because the exam center is closing and you haven't finished.
3. Usually you will have to pay a fee and fill out paperwork - allow enough time on top to get all that done. When I went in, that whole process took at least 45 minutes because I had to wait in line, fill out forms, walk over to a different building to pay, return to the testing center, sign some more forms, then wait for them to set up the computer station for the HESI. It may be different for you, but I think that process is quite common so be prepared for that and build in the additional time. I can't recall the exam length, but I want to say it is either 3 or 4 hours long, so you need to give yourself at least 5 hours from arrival.
4. A lot of the questions are pretty straight forward, but don't rush through because you're panicking about time. Read the questions slowly and carefully.
5. Brush up on grammar
6. Brush up on fractions and percentages
7. There is an online calculator (at least, there was for me) - you won't be allowed to take anything into the room with you
8. Bring a sweater in case you get cold. The room I was in was freezing (to me) so I was so glad I brought a hoodie to throw on.
9. There are a bunch of online *free* practice tests and quizzes - use them!
Best of luck!