One thing you have to remember when you start a garden is that you have to plant for when the plant is mature. What I mean is, don't overcrowd your plants, plan your space for when the plant is mature.
Have you thought about knockout roses? They are great. My neighbor has them and they are so easy and pretty. You do not have to prune them or anything. They are disease free and you can cut them back without harming them. They bloom from spring up to fall. I just put about 6 of them in last year so I am hoping that this year they will really be a show.
As for the mums, you should keep them trimmed until about July. You should keep pinching them back until July so they can get bushy. I know it is hard to pinch the blooms but trust me, in the fall it will be worth it.
Grasses are really easy also and they bloom in the fall. When I did my landscaping, I tried to have things that bloom in succession. I've been pretty successful, just got to fine tune it. For early, I have crocus, daffadils, grape hyacinths, hyacinths and tulips. For mid spring, I have tulips, creeping phlox. For late spring (April-May) I have azaleas, peonies, irises, hostas (they blooms in summer). For the rest of the season I have things like different color coneflowers, roses, day lilies, grasses, artesmia (sp), asiatic lilies; pincushion flowers, sedum, poppies, abelia, guaras, perennial geraniums, guaras, canna lilies, carnations, turtle heads, black eyed susan, alesstromeria, shasta daisy, foxglove, allium, blue eyed grasses, liriope (blooms in the fall), sedum, hydrangeas, gladiolas, russian sage, butterfly bush, lilac-although it has never bloomed), several roses of sharon, calla lilies. This is just to name a few. This is 3 years in the making and I've changed things around a couple of times. I also put the typical annuals in like impatients and petunias. In the fall I have the mums, stella oro lilies, annabelle hydrangeas.
Can you tell I love to garden?!!
I am trying to convince hubby to make a couple more flower beds so I can put a crape myrtle in. He keeps saying that I have more than enough now (and so do my neighbors-the joke around the neighborhood) lol.