Always a beginner

Hi Sandra and welcome to BT.  Your post suggests you're somewhat familiar with BT and a mentor group is a good idea to start with.  I also like to read race reports for information and inspiration.

If your swim is horrible then check into groups to swim with at your pool or get a coach to help you get a good start.  It took me about 6 months of steady 3x/ week swimming to be able to complete my first 1000m swim and I was swimming with a group and a coach.  If you're younger and stronger you'll likely pick it up fairly quickly.  You will also need to practice swimming in open water to get comfortable with it.  Just make it fun for you.

Don't be afraid to ask questions here.  It really helps to speed up your learning process.  For yourself, locally, you should volunteer at races to further learn how triathlons work and to meet likeminded people.

An Oly (5150) is a fun distance but consider a Try-A-Tri and/or a Sprint first as they will add to your experience and teach you how to deal with the greater distance races.

Have fun and never get discouraged if you have a bad day, bad workout or bad race.  You will have those but when you get it all worked out it is a lot of fun.