the basic answer is what ever you fancy.
basic rules to be followed or not tend to be;
tall plants at the back with small at the front so you get a grade of heights.

you ay want a 'skeleton of shrubs' that add winter interest and then select bulbs and perrenials to flower through the year.

you can go for colour themes or plant types. if you go to the botto of the page and click on B it will take you to a list and borders is there and then you can see the pictures of other members.

welcome to GoY too.

Go to your local garden centre and see what attracts you. Don't worry if you get things wrong first time because everybody does. It all depends on the size of your borders but just go for what you like and over time you will gradually change things and develop the garden to suit your taste.

I suppose a common mistake is to buy shrubs etc that become too big for the space so read what it says about eventual size.

if like me you live in a town and the only local GC is not shall we say on the large side i always make a point of going at least once a month because each time i go theres always something different.

Watch during the day to see what kind of sun exposure you get in the borders, and then read the tags on the plants that catch your eye, so you can match them with their preferred sun exposure. I would also look around the neighborhood to see what others have in their gardens--some you will want to plant, and some you will want to avoid!