My Mac is getting really slow, what should I do?
I'm going on the assumption that you have a MacBook Pro Early 2011. There were no MacBook Airs with that designation and MacBooks stopped production in 2010 and resumed again in 2015.

It would still be helpful to know what specific model you had, however.

It came (usually) with 4GB of RAM, 320GB (spinning, 5400 RPM) hard drive. That's not anywhere "fast" by today's standards.

Here's what I recommend:

Max Out Your Memory.

The max you can put on this is 16GB of RAM.  You can use any of these 16GB kits (2 8GB SODIMMS) or go for this one by Crucial.  I have used this brand in 2 iMacs and 4 MacBook Pros with zero issues.  The iMac I am typing on right now has 32 GB and I can't tell you how much a difference it has made over the 4GB that came with it.

Upgrade to an SSD

Your 5400 RPM drive is slow.  They picked 5400 RPM over the faster 7200 RPM for one reason and one reason only - battery life.  Adding an SSD will not only give you tremendous speed, but also longer battery life.  I upgraded my iMac just two weeks ago from the 1TB Fusion drive to a 1TB SSD and this thing not only runs quieter, but faster as well.

I am partial to Samsung as they have never failed me in the past.  This is the Samsung 1TB SSD I purchased to replace my failing fusion drive 

Swap out your DVD Drive for Another HDD (Optional)

While you are replacing drives, if your old 5400 Drive is still good, you can use it as a second drive for data, music, pictures;  create your own Fusion Drive or best yet, have an internal drive for super fast Time Machine Backups.

There is a universal optical bay hard drive conversion kit that you can use to install another HDD/SSD in its place.  I have used two of these already, one in a MacBook Pro and the other in a Dell Optiplex 745.  At under $15, it is a super cheap option to add.

How to Install all this "Stuff"

To do any of these upgrades, you will need to remove the back cover.  It's only 10 Phillips screws:



Once you're in, you can replace the drive:



or the memory:



and even install the optical bay kit:



You can get tutorials on how to do this from http://ifixit.com which is where I sourced all these great photos.

Last but not Least

You will want to make a Time Machine backup of your computer.  It will be the easiest way to transfer all your files and settings.  Personally, I like to start with a nice clean slate and just move over data.  Applications I like to re-install.  I have found that my computers are much more stable when doing clean installs rather than restoration of previously installed software.