You don’t say what weight you are but unless you are a whippet then losing weight will have an immediate impact.
Hill reps are best done on hills that you can ride in 2 to 3 minutes when pushing it. Go for a warm up ride of 20mins or so which should get you to the foot of your chosen hill. Now blast up the hill as hard as you can, don’t give up the effort until you reach the finish line. Now coast slowly back down the hill and as soon as you get to the bottom turn round and do it again. Repeat for a total of four efforts and then go for a warm down ride.
To begin with you’ll only be able to do one set of four but after a month or so you should be able to add a second set but leave ten to fifteen minutes between the sets. If you do them correctly you’ll feel like throwing up, it’s that level of intensity you should be looking for.
You should only do one session per week otherwise you’ll overtrain. Try and find several hills with different gradients as training on just one gradient will get you very good at that and not much else – there’ll be one or two gradients that you just can’t get right