A thumb rule is, if you still use Windows XP or Vista, stop here. Upgrade to Windows 10, you're doing yourself and the world a favor.
Most of the time, mods or mod conflicts are the reason your game crashes.
Here's a list of things you can try:
Do you have compatibility mode enabled on gta_sa.exe or samp.exe?
Disable it. It is not necessary. Right click on gta_sa.exe -> Properties -> Compatibility. Here, disable the checkbox "Run this program in compatibility mode for.."
Video / resolution issue
Changed monitor, graphics card, resolution? Delete your gta video settings. Navigate to "My Documents -> GTA San Andreas User Files" and delete the file gta_sa.set
ASI mods / stream memory fix mods
Stream memory fix mods are not needed anymore, this is a built in feature since SA-MP 0.3.7 - delete all your cleo and asi mods such as StreamMemFix and similar, they often cause crashes. Additionally some asi mods like "fix black roads" cause crashes.
Try deleting all the asi mods from your installation, and then every cleo mod.
Model mods still visible after being deleted?
You deleted a mod but it's still in the game? That's because the large gta3.img file is cached on your PC.
Navigate here and delete the entire Rockstar Games folder from it: "C:\Users\woot\AppData\Local\VirtualStore"
(Can't find it? Press windowskey+r and enter the following: "%appdata%\..\Local\VirtualStore")
Still having mod issues?
Delete your entire GTA SA installation. When you re-install it, put it into a different path. For example, usually your GTA SA is installed at "C:\Program Files (x86)\Rockstar Games\GTA San Andreas" - when you install GTA SA, you can select the installation path. Create a new folder, for example on your Desktop called "GTA" and choose this new folder on your Desktop as installation path during the GTA SA installation.
Black screen after alt-tabbing out of the game and back in?
This seems to be a bug with Windows 10 only for some players: Press alt+enter while having a black screen or before alttabing, this will put your GTA into windowed mode and fix it. If you want to, press alt+enter again to enter fullscreen again.
Still crashing with an error code?
The error dialog usually looks like this:
Exception At Address: 0x0071A708
[bunch of more numbers and text]
Take the part after "exception at address" and google for solutions. Most of the time, other people before you had the same problem. i.e. in this case I would google "samp crash 0x0071A708"