I am a beginner at gardening.
My flower beds are lower than the lawn; I want to build beds to the level of the lawn.
My late husband did all gardening, so I am at a loss of what to do. Do I use compost or soil or both?
As a beginner and a new learner, may I suggest bringing a sample of the soil to your local garden center and ask them what they recommend? Your local Agricultural Extension office may offer those services for free or a small charge. They may be able to test it and tell you if you need any particular amendments and how it stands up as a healthy soil.
Absent that, it's hard to go wrong with a good compost mix. Organic matter like compost a great amendment to just about any soil, but the cost of adding it in bulk can sometimes be prohibitive.
If it looks like your garden soil already has plenty of organic matter (i.e. not overly sandy with little structure), regular garden soil would likely be fine. But if you want to virtually guarantee a strong start to that garden and don't know (yet) exactly what you need, you can be pretty confident starting with a mixture of garden soil and compost.
This is a tough question. Please give us a picture or two, Dotty! To build up a plant bed now with the plants already established is dangerous for your plants. Especially if you have woody perennials or shallow rooted plants like Azalea or Daphne. Adding anything on top of those roots could easily kill plants that rely on shallow surface water/chemicals. Even an inch too deep will kill them. Woody trunks if covered with soil, mulch, rocks, moss...will be girdled eventually by bacteria that love the moisture. It would be easier to redo the lawn, cutting the sod out and removing it.
Please send pictures, try to get a wide view so that we can see the slope and levels of lawn versus your plant beds. Are these same plant beds next to your foundation? If so another good reason to cut out lawn to lower the elevation to allow water to drain away from the foundation from plant beds and then out over the lawn.