Do I have good or bad posture?

This is an excellent question. Think about how you sit during the day. Is your head forward or over your body? Are your shoulders and spine rounded forward, or back and straight? Are your feet flat on the floor under your knees or out in front of you? It can be hard to notice the positioning of your body throughout the day, so you'll need to think about it. Another question is, after sitting or standing for a while, do you start to have aches and pains in your back, neck, or shoulders? If yes, then it could be posture related, and be worth getting assessed professionally.

My back aches when I sit for a long time, does that mean I have bad posture?

Possibly. Back ache can be an indicator of bad posture, but not always. Back ache or back pain can be caused by many things and is unfortunately very common. Bad posture itself can be contributing factor to many issues, from headaches to back pain, and breathing problems in some cases. 

To determine what is causing your back ache, several factors would need to be considered. What type of chair are you sitting in and how supportive is it? How strong are your stabilizing muscles and are they properly supporting you? What position are you sitting in? Are there any other contributing factors to your back pain? As you can see, this can be a complex question to answer.

Can I improve my posture?

Absolutely! Posture can be improved with a combination of strengthening weakened muscles and stretching tight muscles. The trick is knowing which muscles to focus on. As a general tip for strengthening, exercises that generally have a pulling motion, like rows, can be very helpful in bringing the shoulders back to improve posture. Pulling exercises focus on back muscles which tend to be weak in people with bad posture.

Weak back muscles in combination with tight chest or front of the shoulder muscles tend towards a more rounded shoulder posture. If this is you, you can try to a back exercise every other day to maintain strong postural muscles, and stretch your chest/pectoral muscles daily. Of course, for a more specific exercise program for your particular posture, you would need to be properly assessed, preefrably by a posture specialist.

Do I have to keep my back straight all the time to have good posture?

In a word, no. Nobody has perfect posture all day. The trick is to practice sitting in a good, comfortable position throughout the day. Set an alarm on your phone or computer to remind you to practice good sitting posture every hour. For five minutes at a time, try to maintain that position. Also, there is nothing better for your posture than to get up and move, so get out of that desk chair every 30 minutes.
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It is natural for you to slowly go back into your default posture because that position is more familiar to your body and requires much less effort for you to stay there. But by practicing a good posture throughout the day, you are slowly creating a new default for your body. You become better at maintaining that new position for a longer period of time, particularly if you also exercise to strengthen those postural muscles.