I use AutoCad and it is excellent but expensive considering that I only use a tiny portion of its capabilities. The benefits are that it is Industry standard. I get provided with building plans, fire alarm and emergency lighting system layouts etc direct from the Architect or contractor when available, which tends to be the norm now.
If you want to get into CAD there are a number of adequte packages available for free.
Get some and try them do a Google or try the attached link.
http://www.structural-engineering.fsnet.co.uk/free.htmWhat you need to ensure is what the file format is in any package you go for. AutoCad uses .DWG files but will also take in .DXF files which is a standard for transfering Cad drawings.
The above site also contains other free software such as PDF converters so if you wish to send documents or plans by email they can be changed into PDF documents which gives you control and prevents any alterations.