Take a breather. It is important to allow yourself some time between the first and second drafts. Jumping into the second draft too early may cause burnout and general writing fatigue, which means you will not finish the second draft or the book. You need time to allow your ideas to congeal into a more coherent story. You also need some emotional distance from what you wrote in the first draft. These things only come with time. Some writers take a few weeks off, some a few months. How long you spend on hiatus is up to you.
Have a close friend or family member look over what you've already written. They can lend an objective opinion on the work and point out plot holes or inconsistencies you may not be able to see. Be sure to choose someone who will be honest with you, but who will also give you affirmation when they see things worthy of commendation. Finding a reader with a good balance of both qualities will keep you from becoming either too smug or too discouraged. They can also keep you motivated to finish.
Reread the manuscript yourself. Some people prefer text-to-speech when going over a draft. You want to see the story with new eyes, so that you can appreciate the good parts and get ready to cut out the stuff you don't need. Be sure to make lots of notes of your thoughts and observations as you read or listen, these can help you out a great deal as you begin rewriting. Make special notes for the parts you really like, this will encourage you to keep writing even when the going gets tough.
Cut the portions that are irrelevant and boring. Chances are that most of the content in your first draft will not make it onto the final printed page. It is possible that your draft could have more things that are cut than remain. If a character doesn't move the plot forward or deepen your understanding of another character, remove them from the story. Do the same with chapters and subplots. Make notations on the original draft about who and what needs to be taken out. Refer back to the really good portions you highlighted earlier and remember that writing is a process, don't allow your self-editing to turn into self-condemnation.
Set your new goals. This can be the number of words you want to write in a day or a date by which you want the second draft to be completed. Regardless of the type of goals you set make sure they are realistic. Don't push yourself harder than you know you are capable of. You want to set goals that you know you will stick to.
Start your rewrite. Some people work from the text of the original draft, others choose to start writing completely fresh, and only refer back to the first draft to get ideas. Whichever system you choose stick with it, and stick with your goals.
Push on to the end. Don't quit until you finish the draft.