Upgrading from Snow Leopard to Lion or Mavericks
To upgrade to Mavericks you must have Snow Leopard 10.6.8, Lion, or Mountain Lion installed. Purchase and download Mavericks (Free) from the App Store. Sign in using your Apple ID. The file is quite large, over 5 GBs, so allow some time to download. It would be preferable to use Ethernet because it is nearly four times faster than wireless.
Macs that can be upgraded to OS X Mavericks
1. iMac (Mid 2007 or newer) — Model Identifier 7,1 or later
2. MacBook (Late 2008 Aluminum, or Early 2009 or newer) —
Model Identifier 5,1 or later
3. MacBook Pro (Mid/Late 2007 or newer) — Model Identifier 3,1 or later
4. MacBook Air (Late 2008 or newer) — Model Identifier 2,1 or later
5. Mac mini (Early 2009 or newer) — Model Identifier 3,1 or later
6. Mac Pro (Early 2008 or newer) — Model Identifier 3,1 or later
7. Xserve (Early 2009) — Model Identifier 3,1 or later
To find the model identifier open System Profiler in the Utilities folder. It's displayed in the panel on the right.
Are my applications compatible?
Upgrading to Lion
If your computer does not meet the requirements to install Mavericks, it may still meet the requirements to install Lion.
You can purchase Lion at the Online Apple Store. The cost is $19.99 (as it was before) plus tax. It's a download. You will get an email containing a redemption code that you then use at the Mac App Store to download Lion. Save a copy of that installer to your Downloads folder because the installer deletes itself at the end of the installation.
Lion System Requirements
1. Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7,
or Xeon processor
2. 2GB of memory
3. OS X v10.6.6 or later (v10.6.8 recommended)
4. 7GB of available space
5. Some features require an Apple ID; terms apply.
If you only want Lion, then just follow the directions to get a download coupon. Mavericks is the current version of OS X. It replaced Mountain Lion which replaced Lion. If your machine does not meet the requirements for Mavericks, then all you can upgrade to would be Lion. But since you did not provide further information I just went and posted both. You don't have to both.