First of all you MUST start taking backups. At present if your disk crashed you would loose all of your data.
A good plan is to take Full backups each day, and Log backups each hour. This can be done by creating one maintenance plan for the daily Full backup, and a second maintenance plan for the hourly Log backup. Do not use any options to shrink your database in the maintenance plan, as this will lead to poor performance of your queries. There is lots of advice in Books Online and on the web about database maintenance – it is worth taking an hour or two to read and learn more about what you can do.
There is no method with the software provided with SQL Server to selectively undo unwanted data. However, there are a number of third-party products that can analyse your log file and generate SQL statements to remove changes according to set criteria. In this way you could remove all changes made by user XXX between two dates, etc. The hard bit in this type of selective deletion is working out what criteria to use. You should definitely try this on a copy database before changing your live data. Especially as you are very very close to loosing what data you currently have.