Let me start off by giving the disclaimer "I'm not an expert, but this is what has worked for me." Ok? That takes the pressure off me to solve all of your homeschool high school woes, yet still gives me an opening to share. Here we go!
Know your child. Each child is different, and you need to have an understanding of where your student is and where they are heading. Not everyone will go to college, for that matter, not everyone SHOULD go to college. Determine if you need to prepare them for college, trade school, military or going directly into the work force.
Prepare Accordingly. Based on the information you have from above, begin to plan out an overall view of their high school time. If you aren't 100% sure about college, but you want to have your student ready "just in case", then take a look at your state or local school districts graduation requirements. This will give you a good baseline to what classes are generally expected for graduation. You can then tailor your plan to fit your child's interests, while keeping college preparation in the realm of possibility.
If you know that your student is college bound, then check college requirements at their preferred school(s) and make sure you get all of the required classes covered. Again, looking at your state or local school district's graduation information will show you what an "advanced" or "AP" diploma would require, and this again is a good baseline to make sure your student is competitive academically when applying to colleges.
For trade school, or going directly into the work force, begin looking at your local options first. Some areas will allow homeschooled students to attend the vocational technical schools. Apprenticeships are also a great opportunity, and often are paid positions. If there is nothing locally that suits your needs, then go to our friend the internet and do research, research and more research. There are more options than you can imagine out there, but sometimes you have to do some digging to find the right one.
Realize that consumer math and business math are great options for high school. Don't make yourself crazy with high end academics if you know your student isn't going to need it. Also, realize if later in life they suddenly decide they want to further their education, they can do classes on their own, online or at community college.
Make a general outline. Don't make yourself crazy by trying to pick out all four years worth of curriculum at one time. Do figure out what classes you want/need your student to take each year. Donna Young has great, free resources on her website for this. Or you can just write it on notebook paper and go from there.
Document everything. A simple spiral notebook can be a great help here. Keep track of extra curricular activities, awards, reading log and curriculum vendor information. Basically this notebook will be a history of your students high school career. Write it down now, because you will forget, trust me. This will help you create transcripts, complete college applications and even job applications.
Transcripts. This one word strikes fear into the heart of many a homeschooling parent, and it shouldn't. If you're documenting things as you go (see above) it makes transcripts much easier. However if you still are worried about pulling it all together into something usable, there are several great online option. I use My Home School Grades to track everything, then with a click of the button I have a transcript printed.
Testing. PSAT, SAT...the dreaded "testing" is another area that causes panic in some parents. College Board online gives you all the info you need to prepare, deadlines for registering, and get your students test results.
Lastly, remember this is a marathon, not a sprint. You do not have to do everything today. But, remember the easiest way to eat an elephant is one bite at a time. The smaller the bite, the easier it is to digest...likewise don't wait to the week before college applications are due and try pulling it all together out of your magic hat.