I'd bring it to a computer shop if you are doubting yourself and you have important data on it that is not backed up etc.
If you want more space safely, you could enable drive compression on the hard drive and gain more space....only downside is that all data has to decompress from its compressed state so it slows the computer down to gain the storage space.
Best option is to have someone migrate your data to a new larger HD, but if you dont want to spend any money and dont mind the computer slowing down, you can use drive compression which is very easy to turn on for more space.