But the good news is that the more you crochet, the more you will develop a feel for how crochet works. With each item you crochet, you will gain experience and knowledge that can be applied to future items. And there are a few tips I can share that might help.
Unless you are very ambitious, I would suggest you start out with simple items with little or no shaping for your first designs. I wrote a post a few years ago about designing a wash cloth:
This might be a good project for first time designers.
If you are going to design an item which has shaping, take the time to learn how to make both increases and decreases in the stitch pattern you will be using before you begin. It will save a lot of frustration once you get started.
Round items are a bit more challenging because they must lay flat (usually.) This means you must add just the right amount of stitches to each round. Here is the pattern for a basic circle in dc that I often use:
|I used the basic circle pattern to start this Watermelon Potholder design.
Rnd 1: Ch 4, 11 dc in 4th ch from hook, join with sl st in top of ch-4. (First 3 chs count as first dc; 12 dc made)
Rnd 2: (Ch 3, dc) in first st, 2 dc in each st around, join with sl st in top of ch-3. (24)
Rnd 3: (Ch 3, dc) in first st, dc in next st, (2 dc in next st, dc in next st) around, join. (36)
Rnd 4: (Ch 3, dc) in first st, dc in each of next 2 sts, (2 dc in next st, dc in each of next 2 sts) around, join. (48)
Rnd 5: (Ch 3, dc) in first st, dc in each of next 3 sts, (2 dc in next st, dc in each of next 3 sts) around, join. (60)
Rnd 6: (Ch 3, dc) in first st, dc in each of next 4 sts, (2 dc in next st, dc in each of next 4 sts) around, join. Fasten off. (72)
If you look at the pattern, you will see that the first round has 12 stitches, and each of the following rounds increases by 12 stitches. This circle will lay flat, and it can be made as large as you would like, just by continuing to work rounds which increase by 12 stitches each time.
When working in a pattern stitch, it is up to the designer to figure out how many repeats are needed to make a circle work the way they want it to. Pineapple designs have been my major challenge. It is sometimes hard to know when the next pineapple should begin, and it is often just a case of trial and error.
As far as coming up with ideas for designs, I pretty much get them everywhere. I have a collection of vintage children's books that I have been going through lately, looking for inspiration for some Lovely Ladies Doilies which will feature little girls. You might try combining some of your favorite stitches to create your first designs. For example, you could work a square out of shell stitches, then add a lacy border of love knots to create a lovely shawl. You can also make items based on your lifestyle or personal likes. It can be a great learning experience to design a shopping bag made out of plarn or recycled yarn, or dinosaur hat for a favorite little boy.
One thing I want to point out if you are designing clothing, is that you should always keep up with current trends. No matter how cute your shrug or poncho design might be, the pattern is probably not going to sell very well right now because both of those items have faded in popularity. It is a good idea to browse Ravelry and Etsy to keep up with developing trends. And remember, any time a celebrity is seen wearing a crocheted item, there is a pretty good chance that type of item is going to become popular (remember Martha Stewart's poncho?) Of course you can stick to the basics, like scarves and hats, but even those basics can change in style from year to year.
I hope this series of articles has helped a little, it has been something I have wanted to do for a while. I would like to encourage everyone to try designing. It can be a challenge, but it is worth the effort.