Hello,

First of all when it comes to appearing for UPSC you need to be 21 years old, passed/appearing graduation in any discipline from a recognized university, there is no minimum aggregate required in this case, now you're seeking government job opportunities after 10+2 , you can appear for SSC CHSL as this is based on 10+2 level, and if you crack this exam then you have to manage your studies along with preparing for UPSC if that's what you eventually aim for, you can also try for state level jobs such as clerk/assistant, various government organizations also recruit candidates for these posts,there are recruiments done by railway as well for clerks in various departments, there are ssc mts, if you have stenography skills then you can appear for SSC Stenographer,  you can stay updated by visiting job portals regarding this, the scope here is limited compared to once you complete your graduation, as the opportunities increase further after graduation in various fields such as banking/insurance as well which offer quite handsome pay scale, and your other question regarding securing good marks in 12th standard, consistency is the key to excellence, hence it's important that your maintain a routine else procrastination might take place, plan your schedule and accordingly assign the time to be spent on each subjects, have a conceptual understanding first, NCERTs books are written in simple and clear language, hence refer to these first and later on move towards standard books, g though previous year paper/sample paper in order to know paper pattern, marking scheme, and other important details which will help you in qualifying the exams with good marks, take care  of your health and mind by doing yoga and meditation on regular basis, in case you want to know anything else , you may also search our site for any detailed information regarding the same.