Diagrams
In EUDP we generally uses UML and SysML diagrams.
Tutorials and Software
For Tutorials Please use your favourite search engine to search for "Tutorial UML" or "Tutorial SysML".
UML
Over at Tutorialspoint you can find excellent tutorials in general and also for UML.
Dia is a rather simple, yet effective, tool for creating diagrams. Binaries are provided for major operating systems over at Dia downloads page.
If you are running Linux you'll just have to install it from the repositories provided by your specific distribution. E.g. in Centos execute
]$ sudo yum install dia
Another free tool in most Linux'es is Modelio, which also supports UML diagrams.
If you are running KDE as tour desktop kdesk-umbrello may also be a useful choice for you. If you are running on top of Gnome just search for umbrello
SysML
SysML is a bit harder to locate on-line resources suitable for self study.
Eclipse hosts Papyrus, which is a SysML and UML drawing tool of high quality, but maybe not that easy to get your hands on. There is a guide in the project space on Eclipse.org, which can be used for getting started with SysML and UML Diagrams. Downloads are located in the same project space.
Block Diagrams
Block diagrams can be made in any text editor or drawing program like Libre Office Writer or Draw.
Dia and Papyrus can also be used.
Flow Charts
Flow charts are often used as a method of illustrating a flow in a piece of software. Find over at Tutorialspoint a brief introduction to flow charts.
Dia is a useful tool for drawing flow charts.