Getting started with DBpedia

The answer regarding your first question,actually SPARQL is a query language not a software and you can write your queries here https://dbpedia.org/sparql.

In-order to obtain countries ,their capital and respective population :

PREFIX dbo: <http://dbpedia.org/ontology/>
PREFIX dbp: <http://dbpedia.org/property/>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>

SELECT  min(?country_name) as ?Country_name min(?capital_name) as ?Capital_name min(?population) as ?Population
WHERE {
?country a dbo:Country.
?country rdfs:label ?country_name.
?country dbo:capital ?capital.
?capital  rdfs:label ?capital_name.
?country ?p ?population .
FILTER(?p = dbo:populationTotal || ?p = dbp:populationCensus). 
FILTER NOT EXISTS { ?country dbo:dissolutionYear ?year }
FILTER langMatches( lang(?country_name), "en"  ).
FILTER langMatches( lang(?capital_name), "en"  ).}
GROUP BY ?country_name

For your third question, this is an example solution :

SELECT  distinct ?link ?person_full_name ?birth_year   WHERE { 
     ?link a foaf:Person. 
     ?link ?p ?person_full_name. 
     FILTER(?p IN(dbo:birthName,dbp:birthName,dbp:fullname,dbp:name)). 
     ?link rdfs:label ?person_name .    
     ?person_name bif:contains "abdul" . 
     OPTIONAL { ?link dbo:birthYear ?birth_year .  }
     FILTER(langMatches(lang(?person_full_name), "en"))
     }
     LIMIT 100

Check out About DBpedia and Using DBpedia.

Also

• SPARQL interface -- the SPARQL environment is already there
• Faceted Browser -- another way to explore the data
• Graphical SPARQL Query Builder, iSPARQL
• SNORQL interface