One of the most important things a beginning sewer needs to learn is how to choose a pattern that matches their skill level, and then choose the right fabric for their project. I've seen beginners become frustrated, and abandon their project (and maybe sewing altogether) because things just aren't going well. Often times the culprit is not sewer, but rather the fabric or difficulty of the garment. I can't say it often enough -- what you see as a simple design, does not always translate to a simple construction!
Start with patterns designed for beginners; those will be clearly labeled as "Easy", "Learn to Sew", etc. Before you purchase the pattern, open the envelope and look at the instructions. If you can't make heads or tails of the instructions in the store, it's not likely you'll be able to figure it once you get it home. Click on "Read more" to continue.
Choose the right notions. Choose the type of zipper, elastic, buttons, etc. that are listed on the pattern envelope. If I had a nickle for every time a student showed up with something other than a standard dress zipper....
Whether you choose your fabric first, and then go looking for a pattern, or vice-versa, take the advice of the pattern manufacturer. Every pattern will list suggested fabrics on the back of the envelope. A pattern that is designed for stretch knits will not fit properly when made from a non-stretch woven material.
I generally advise my students to start with woven fabrics such as quilters cotton, or flannel. Once they learn methods, techniques, and characteristics of various fabrics, then they can slowly move on. Look carefully at the weight of the fabric -- a very lightweight, or silky fabric that is intended for lining is probably not going to work out so well for those pajama bottoms you're making.
Mostly, allow yourself time to learn. Take your time with your project. If you work slowly, you'll find that you learn more quickly.