If you don't know where to start then just start anywhere - come up with one or two concrete objectives to work towards. The most important thing is to start. Pick something that looks achievable over 2-3 months and go for it. By the time you have achieved those objectives you will have built up some momentum and also learnt a bit more, so you'll be in a slightly better place to pick your next objectives. As you get better at this you can start picking bigger targets and doing it over longer time frames.
If you're really stuck then try picking an open source project at random and commiting yourself to contributing something to that project in the next two months. That could be programming, it could be documentation, it could be something else. If you're not physically active, then pick some kind of physical challenge too. Completing a 5 kilometre/3 mile run/walk is achievable for most people in that time frame.
The more you do, the more you'll know what to do (and the more you'll realise how much is still undone).
And on a more specific level - if you want to get into a top grad school you will most like have to do well academically. Depending on your course, exams are likely to be a major factor in your grades, so learn how to be good at exams - it is a skill and if you are staying in academia for a while it is one worth learning. What follows is old advice, but not many people seem to follow it. If you're studying a technical subject, then make sure your maths is up to speed. If you are bad at maths it will make any kind of numerical/equation type of exam much harder (and also many things in the real world). During the exam, do three passes through the paper - on the first pass just read it, look for easy questions. On the second just answer stuff that you know reasonably well, don't waste time on difficult stuff. On the third pass go back to the questions you couldn't do and do as much as you can. Even if you can't answer the question write down stuff that you think is relevant, you might pick up a mark or two here and there (but don't go overboard writing down irrelevant stuff, your marker won't thank you for that). And practice, practice, practice. Get hold of old exam papers and do them (timed). Get hold of the answers to the old exams if you can and study them. Often the questions are set by the same person each year and often they aren't very imaginative at coming up with new variations.