Open a command window and drag stsadm.exe onto it and use the following command to add the solution to the solution store.
stsadm -o addsolution -filename c:\solution.wsp
You can also use stsadm to deploy the solution but here I am going to describe the point and click method using SharePoint Central Admin which does the same thing.
Open SharePoint Central Admin and select the Operations tab.
Select Solution Management.
Click the link to the .wsp solution you just added.
Click Deploy Solution - If the solution is not globally deployed select the web applications you want to deploy to.
The solution is now deployed to the web applications you selected or to all web applications if it deployed globally. This is not the end of the deployment process though. If the solution contains a web part there are still a few steps to go before it will be displayed on your site. To add the web part to a page on your site do the following:
Navigate to the site collection where you want to place the web part and go to Site Settings and Site Collection Features (this assumes that the feature is scoped at the site collection level, sorry for making more assumptions that you understand scopes but that is another post and something that other people have already written plenty about if you want to find out more).
Activate the feature that contains the web part that has been deployed in the solution. Next, go to Site Settings and Web Parts and click New.
Select the web part to add it to the wbe aprt gallery for that site collection.
The web part is now ready to be added to a page in the usual way - edit page, add web part to zone, browse to web part.