Hi, welcome to SRK!
Please don’t get discouraged. Nobody is good when they first start out. Sakura is a tough beginner character because she’s really dependent on 1 & 2 frame links to get most of her damage. If you try to play her without using that stuff, you’re basically better off playing Ryu or Cammy.
Still, if you’re determined to keep using Sakura, I am sure you can start winning by improving at the fundamentals of Street Fighter. Check out the Vesper Arcade SSF4 tutorial, read the Sakura wiki page, start understanding the distances at which you can poke with your normals, especially MK/HK, and learn which normals can link into each other (also on the wiki, under Blockstrings). Use some basic blockstrings for your pressure and learn to identify when you land a hit, so you can cancel into Hadoken/Shouoken. Don’t worry about any more advanced combos for awhile.
After you’ve got that down for a week or so, learn how to FADC properly. You want to practice FADCing her EX Shouoken into s.HP, which will open up a lot of combo possibilities for you when you have 3 bars of meter. You can also practice linking LK Shunpukyaku (her spinning kick) into s.LK. It’s a 2F link, but if you get it down, you can do stuff like: LK Shunpu > s.LK > EX Shunpu > Sakura Otoshi or Ultra 2. Very useful. Once you’re comfortable with all of that, you can start learning how to plink and do her 1F links if you want to.
And keep in mind that even though my advice was for Sakura, you can use this same advice with any character you choose to play. Read the wiki, learn your spacing, then basic special move cancels, then tougher and tougher combos gradually. Good luck!
(Oh, and as for the CPU question: sure, that’s totally fine! Don’t get into the habit of playing against the CPU forever or you’ll develop bad habits, but you can use it while you’re still learning a character before you hop online. It’s also useful to train your combos against a target that fights back.)