I wouldn't till in whatever the store tells me to;
too much likelihood of getting advice from someone who doesn't really know.

I also wouldn't till in anything made by Miracle Gro or Sevin or Monsanto or Ortho;
gardens do beautifully with compost & mulch & maybe a little green sand or lava sand.

Thanks for trying to help, but...

OP said "1) Send your soil to the County agricultural extension service which may send it to a state lab. They will tell you what your soil is deficient in"

A number of states have "county agricultural extension service" affiliated with the state university that DON'T do soil tests, nor do they send to a state lab. Oregon & California are just two. Instead, must go to a private soil test lab.

Don't till potting mix into soil -- it wastes time & dollars.
Also, Don't till top soil into your soil -- it wastes time & dollars.
Instead, mix in at least 2 inches of compost throughout the garden bed every time you plant.

Tips for newbies, is it?

Start small. If you have only a few hours free on a weekend till only what you can keep tended in HALF that time. Otherwise the weeds will win every time. Sad but true.

Keep the veggie garden near the house so it's easy to gather foods and herbs regardless of weather.

Hard paths beat fancy grassy high maintenance paths every time.

Keep your favourite tools in excellent order and instantly available - including a container for weeds. Always be ready to pull weeds.

Keep your garden clean. Clear away debris and decay ASAP to prevent the build up of pest plants and animals. But keep the neatness freak stuff for indoors.

Make your garden a friendly place for birds and beneficial insects. Be totally prudent with sprays unless you love moving pollen between plants. Bees and moths and birds will always do it much better than you can.

Mulch. Frequently. And build your soil with well-made compost in every season and with every planting you make. (It's cheaper than chemical fertiliser and endless hassles with pests, and hefty water bills.) The very best soil amendment you could ever apply.

Capture autumn leaves and put them in to compost. If you can, mix them with grass clippings and finely cut twigs so they don't form squidgy clumps. Let the air in and the rain/snow.

Get to know your soil. Some soils can be worked at any time. Others will be ruined if worked when too wet or cold. (Various clay-based soils come into this category.)

When you start - grow plants for your area/zone. And find alternatives for achieving The Look you're after. Cute English gardens look well, um, silly in dry areas. Use a different vernacular such as Mediterranean.

Know when to quit struggling with a particular plant. (Before the bank manager rings.)

Wait for at least a week after reading a gardening magazine before scampering out to buy That Gorgeous Thing. It might be a monster in disguise. Google it. Ask. Peek over the garden walls to see if it grows in your area.

Tips for newbies, is it?

Start small. If you have only a few hours free on a weekend till only what you can keep tended in HALF that time. Otherwise the weeds will win every time. Sad but true.

Keep the veggie garden near the house so it's easy to gather foods and herbs regardless of weather.

Hard paths beat fancy grassy high maintenance paths every time.

Keep your favourite tools in excellent order and instantly available - including a container for weeds. Always be ready to pull weeds.

Keep your garden clean. Clear away debris and decay ASAP to prevent the build up of pest plants and animals. But keep the neatness freak stuff for indoors.

Make your garden a friendly place for birds and beneficial insects. Be totally prudent with sprays unless you love moving pollen between plants. Bees and moths and birds will always do it much better than you can.

Mulch. Frequently. And build your soil with well-made compost in every season and with every planting you make. (It's cheaper than chemical fertiliser and endless hassles with pests, and hefty water bills.) The very best soil amendment you could ever apply.

Capture autumn leaves and put them in to compost. If you can, mix them with grass clippings and finely cut twigs so they don't form squidgy clumps. Let the air in and the rain/snow.

Get to know your soil. Some soils can be worked at any time. Others will be ruined if worked when too wet or cold. (Various clay-based soils come into this category.)

When you start - grow plants for your area/zone. And find alternatives for achieving The Look you're after. Cute English gardens look well, um, silly in dry areas. Use a different vernacular such as Mediterranean.

Know when to quit struggling with a particular plant. (Before the bank manager rings.)

Wait for at least a week after reading a gardening magazine before scampering out to buy That Gorgeous Thing. It might be a monster in disguise. Google it. Ask. Peek over the garden walls to see if it grows in your area.

Get yourself a trustworthy gardening book that's right for your area and the sort of gardening you want to do. Preferably one that gives you month by month advice. Even very old ones are fine, though the plant varieties could be out of date. Gardening basics mostly stay the same. (But if they're talking about DDT and nicotine you know you're back in the best-forgotten past!)