For just a personal garden, that I just want a few plants, I find seeds or get fruit from the wild or grocery store.
The guide gives a lot of info, but there are tips here that you can find helpful. One tip is to fence in your garden. Mainly to keep the zombies (if you have SN) from eating your veggies. They don't eat the fruit from the trees. Another reason is if you have pets it will keep out the wildlife. Horses, deer and others will eat the veggies. Third, I do as Joria and leave the sprinklers on (I have found them in outdoors, click on the grill, then click on the football and you wil find them there) but my sims want to play it all the time; forgetting to eat, sleep or anything else. So, I put up a fence but no gate until I am ready for them to take care of it. That means having to go to the buy mode to get a new gate each time you plan to send them to the garden, but for me it was worth it.
If you have WA, you can travel to the other countries and find seeds or get fruit or veggies at the market that can be planted.
Or, as suggested, use the cheat to get the plants. I have found that if you fertalize your garden, you will get better quality. Even with the ones from the cheat, when I gave my plants a high quality fertalizer (garlic, for example) they would go from normal to very nice.
I am working on a garden to put at the Exchange, or the Swap Shop (I think I have that right) and you could download it and have perfect plants. If you don't have the expansion packs, I think that section of plants will either just be blank or have something ele planted there, not sure how that works. I have plants from WA and SN in my garden. Set up for alchemy and for a nectary along with the regular plants.