If you're coming from Cisco (I did too), you'd be familiar with using ACLs and route-maps to apply additional settings.
The same concepts apply in Juniper of course, but the way they're implemented is different.
On a Junos router, you have a firewall filter (like an ACL) and route policies (like route-maps)
Firewall filters match 'interesting' traffic using conditions like src/dst IP, port, protocol, and others.
They are then used in the routing policy to achieve your goals (like AS-Path prepending, changing local preference, etc).
Here is what I recommend. Go to https://jlabs.juniper.net/vlabs/ and log in. Choose a suitable topology, and start it up.
Then, start configuring BGP, firewall filters, and route policies. There is more information in these links:
https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/routing-protocol-bgp-security-configuring.html
https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/firewall-filter-ex-series-cli.html
https://www.juniper.net/documentation/partners/ibm/junos11.4-oemlitedocs/config-guide-routing-policy.pdf