Clean up and groom your dataset in Excel (or similar)
Try to get the whole dataset into one tab
Clean out all the decorative extra rows and columns that make an excel document look great, leaving just one set of column headings and no indents, asterisks, or merged cells.
Save the cleaned up file someplace easy to find
Get a colleague to review it - just to make sure you're not using any social security numbers or other category 3 or 4 data.
Upload your first dataset
Use the Create a new Dataset button on your data.wa.gov profile page and follow the wizard steps to find the file, upload and check it. Fair warning: it will take about 10 minutes to upload a file with thousands of rows.